Hadn't seen this mentioned but perhaps I missed it, more plumbing hardware than electronic, but perhaps useful for others.
Having tried a variety of plates and heat-sink compounds and tie-wraps to hold these little chips in position on small sections of metal pipe (my rads are fed with fused plastic pipe), I came up with the solution of using pipe clips, the cheap plastic variety from DIY stores and hardware merchants.
The first problem hit was with attaching plated sensor the the boiler clamp nut, it was off by 10c and no way could I get a more accurate reading, presumably angle? Finally found a pipe clip which got hacked in half to fit in the tiny gap in the photo... Some hacking with a Dremel, some soldering to a pin connector, some fiddling around with epoxy and voici....

The gap
The tie wrap in the top of the photo is the original plate mounted version which was "off"..

Having scored on that front with more reliable readings I looked at the roving radiator test kit and realised the same solution might apply so epoxied the chip on the end of some telephone line in a 22mm clamp and it works a treat. Cable and chip fit neatly into the screw-hole, flood with epoxy and done. The pipe it fits is 21.5mm so tighter than a duck's rear end...

As said, a tight fit to a twinwall radiator feed pipe, and a complete swine to get back off even with pliers, but no broken wires or oopsies to contend with, and so much faster to fit.
For those looking to measure metal pipe temperatures, possibly an unusual if fiddly solution, but no need of tie-wraps in the latter case...

Not so much a MySensors build as an example of how even the most basic information can inform changes for the better, in this case space heating.

The system here is fairly basic, an array of DS18B20s, some ultrasonic tank probes and a gas reed sensor, temperature is updated every 5 minutes, the gas updates every 0.05m3...
With winters here down to -20, the first priority last year was insulation, and even though a modern house, the gas bills essentially halved over the year, effectively funding not only the insulation, but replacement axial radiator valves and thermostat heads (Heimeier) to replace the typical arrangement of unknown origin, with spare... But now the MySensors impact..
This autumn's attention turned to the central heating unit, a modern combi unit of good manufacture, installed by a 'certified' heating engineer, but aside what little I knew about condensation boilers and the steep learning curve that followed, I was bemused by the return from the radiator loop almost burning my finger within 10 minutes of the system being fired up. This did not make sense for what I understood of a condensing boilers, which compelled a look inside for the first time, the manual and some googling.

The boiler is a 25kW combi with minimum output 7.6kW, the radiators account for ca 13kW at Delta 60 set for 15c drop (previously set ca 20c drop), settings since day one were 65c and the pump was set at max output of 3, last year's -20 resulted in 13.5m3/day gas consumed, not crazy by historical records, but hmmm.
So now comes tinkering with data from MySensors via Domoticz to inform...
Currently the boiler is set at 55c, the pump is on Low (40 v 84w), but the results are surprising - Slower rising temperature when ON, 42 minutes v 25, but gas use dropped from 0.75 to 0.6m3, but here's the kicker from that longer heating time, not only less energy used per cycle, but longer and thereby fewer cycles per day. Current evaluations are between 15 and 20% savings, so thank you to all the MySensors community and contributors..

Surprised I'm not bald will all the hair pulling and failed attempts to get there but after a year it is finally working...
The task was simple enough, to measure fluid depth in two tanks, one a sewage holding tank, the other a bulk water tank. Just figuring out which method worked with which board was an exercise in frustration, but the JSN-SR04-2.0 turned out the only one of the two I could get to reliably work.

Despite the industrial look, these waterproof light switch enclosures proved ideal, as the batteries in holders fitted beneath the clear plastic. A little hacking out of the switch 'fingers' with a Dremel gives plenty of space in the base area.
Battery holders are held in plastic conduit which are hot glued to the back of the box poking through the front section aperture.

The top case is a 3v3 WhisperNode with onboard RTC and RFM69, and tucked under the spaghetti in the top left back corner is a level converter. The RTC wakens the Node every hour to initiate readings, and despite a few hiccups seems to be reliable. The WhisperNode is allegedly capable of draining the 2 AA cells down to 0.9v, time will tell...

The lower box holds the 5v ProMini, a small latching relay, and the ultrasonic board, the button type ultrasonic (like parking sensor) is mounted on the end of a 3/4" collar to 1/2" galvanised steel tube drilled centrally through the roof of the 1.34m x 1.67m tank area.

Connection between the cases is via Cat5e in 16mm conduit, 2 cores for I2C, 3 cores for relay control and 3.3v power, 2 cores for 5v and ground to the level converter.

Despite worries over echoes in the resulting space, the tank was emptied last night, and the reading is coming in with the occasional glitch at 1620mm depth from the head, which I believe has a cone angle of 45 degrees.
The Node control of a Relay by On/Off pins to Mosfets may seem wasteful, but I had them anyway. The 5ms on/off of this small latching signal relay boots up the ProMini and seems reliable.
Still to incorporate a WDT/Reset on the 5v as it periodically hangs, and a time limiter on the Node to complete the task, but glad it is all all working semi-reliably, even if some of the programming side I still don't understand...
The 5v tests for 2 consecutive identical readings, the 3.3v tests that it fits within acceptable range, and if not calls for a further reading.

Once delivery of a second identical board from China is done, the water tank should be a lot easier. This one is the more important, as ran out of water previously due to corrosion of the level probes in the tank (3 core cable and short bolts) which stopped the borehole pump topping up the tank, and the hidrofor hit it's cut-off electrode. The level monitoring will give early warning to go investigate long before the bulk tank is 'empty'...

Postscript 12/7/18 - Water tank worked flawlessly from first deployment, but led to concerns over dropping battery voltage.. The problem appeared to lie with the Master sinking power through the I2C lines, but a routine to null the SDA/SCL lines prior to sleeping seems to have halted the decay.
One of the unexpected results of the Sewage Tank deployment was detecting a dramatic rise in level during a heavy storm (85mm in 24 hours) initially thought to be ground water intrusion.
Excavation to the incoming pipe connection to the tank found a round pipe in a square hole (not exactly an unexpected building practice here, now packed out and sealed.
Losing 30% of storage capacity would otherwise never have been detected, so it the deployment has already paid for itself handsomely...

@pepson This reminds of that famous quotation "Any ship can be a Minesweeper..."
"Once...."

@neverdie Would this work ?

@sundberg84 Yeah, this brings back memories, particularly of quite a few hangovers.
I used 3 and four wide fireclay pipes for bottle storage, they were used for ducts back in the day before plastics took over, cable went optic fibre. I noticed recently here they are back in stock in stores as....yep...wine storage racks.... but wacko pricing relatively...
The beauty of the fireclay was it's slow temperature and humidity change, and this formed the bulk of the thermal mass in the cellar I made below the floor.
I used a 150mm glazed ceramic drainage pipe dug in under the garden as a loop (rope caulked joints - anybody remember them), from memory down about 1.5m, both avoiding frosts and baking sun, don't think the temperature varied over a degree all year round, the ground acting as a massive heatsink which maintained a constant temperature all year round.
Only when the hatch was opened did the temperature jump, a small fan kicked in when the hatch was closed and ran for 30 minutes, a second contact switched on the lights and shut them off, long before LEDs were so prevalent, old reliable (until you were depending on them) incandescent bulbs.
That was it really.
Biggest problem I found was humidity in the early days, probably the fresh construction...

@tbowmo Hah, you were lucky, I finally upgraded from the monochrome to colour...

@sundberg84 Easy, one is disconnected... At 90 degrees?

Finally the round-tuit UPS got built after the last power cut clobbered the Controller/Gateway system, lesson learned...
Meanwell AD-55A, 7.2Ah Acid gel battery, two usb 5v buck converters, a spare socket for raw volts, and a 15 euro IP66 box from the local shops.. Some drilling and filing to the lid, couple of brackets, spare bolts, banding, soldering and hot glue...
A bit bulky, but disappears in a void under the stairs, two tiny drill holes let the buck converter leds shine threw...
No monitoring as yet, but sailed through a power cut this morning and the Pi didn't skip a beat.. First up is the Pi's RTC then can put the cover back on the Controller...

First of the winter projects done, just the programming to finish and replacement temperature chips to get (broke the legs off both DS18B20s through clumsiness).
With mains/battery backup and RTC, it will record boiler start/stop and run times as well as feed/return temperatures, and as an aside the inevitable power cuts which plague this part of the world.
The MCU plugs into a socketed backplane hot-glued to the back of the box should removal prove necessary, but quite pleased it is sturdy and all fitted into a slim 25mm deep standard (cheap) box.

@alec44 Induction ? Pressure switch with a roller ?

@siddarth-sid Perhaps irrelevant since form-factor featured in your post, but...
I put in a UPS which featured on Instructables a while back based on a MeanWell AD-55A PSU, which is still working since 2018. It's neither not neat or compact but has sailed through multiple power cuts suffered here with aplomb. Simple enough to wire up, no processing required, the SMPS does it all.
Never did get round to powering the router from it as originally intended, but ain't dead yet...
I think this links https://forum.mysensors.org/post/100533

@KevinT Hopefully it helps, the Sleepy Dog implementation was mentioned here https://forum.mysensors.org/topic/6908/adafruit-feather-m0-rfm69hcw-with-mysensors/6?lang=en-GB
I'm still at the screwing around stage, even managed to get a puff of magic smoke when I removed the battery while still on USB power. Oops... Presumably I've blown the battery charging circuit, but it's still functional for fiddling with and testing otherwise.

For mains plus battery backup nodes I'm less concerned over sleep than continuity through power cuts, the extra pins over the 328 are a godsend. If frugal consumption is possible so much the better.

The recommended Adafruit library and FeatherM0 board definition are fine for Blink etc., but anything MySensors related fails to compile, so will try the Sensebender definition next and hopefully make some progress.

@KevinT Suggest a keyword search - I stumbled across a post which mentioned "Sleepy Dog" working for SAMD0, whilst trying to figure out how to similarly deploy a Feather RFM69.
I'm still a novice with this device, but I do recall the author saying Adafruit's "Sleepy Dog" worked, and IIRC there was some discussion over the power consumption not being spectacular, but best find and read with fresh eyes.
The stumbling block for MySensors appears to be that the only SAMD0 implementation developed was for the Sensebender Gateway, which logically was never intended to go to sleep.

@rejoe2 Anti-freeze.

@skywatch Just to add to your points on 254 max children, definitions, presentation, etc., which may help @mrmuszynski at some stage.
I've an array in a sketch with 14 DS18B20 addresses, all paged in turn using the one wire protocol on a single pin, so it is entirely possible to hit the 254 children limit before looking at I2C devices.

The beauty of the DS18B20 array is individual chips are "defined" sequentially, hence making presentation and fetching values available to a FOR loop, which makes for a highly compact sketch.

@CrankyCoder A third the volume I'm looking at, but in your case no pump issues.
Perhaps you might have a look at some of the less industrial sensors for what you want - a little differential sensor from Digikey on the end of a some poly pipe to a bib connex on the water line might give you all the info you need, ready compensated against atmosphere...

@CrankyCoder Sorry, didn't see this post appear - Yes it could work IF you can get a sensor of sufficiently low value.
The 5psi sensor was the smallest stud type I could find cheap - ca 6mm steps equates to ca 15 litres here, a 2psi would be even more accurate at a range would be 1.4m.

Interconnected barrels will all balance hydraulically, so with a single sensor on the common discharge you could report a per barrel value and do the multiplication for the barrels in service.
Do remember this is a static hydraulic value, velocity at the point of sampling has an effect, hence why readings are inhibited when the pump is running, since the sensor's fitted to the suction line.

This is the plot I'm getting - the 40 litre variation is presumably drift from the sensor, atmospheric pressure changes, or something I've missed, but not critical at less that 2% of tank capacity.
When the borehole pump kicks in to fill the tank the graph is a straight inclined line on the graph, so overall happy with this.

Would any users of these devices be able to advise whether the power transition between USB and Lipo is per a UPS ?

I'd looked at these initially but decided on 328 devices which at that time I knew worked with MySensors.
Only 4 remaining pins available on the 328 was one limit, the other was power transition - From USB to battery no problem, but restoration of USB power caused a reboot - In reality only a problem for one Node on dual power sending in water meter readings.
Having now damaged two of the 328s (don't ask), now looking for replacements with a little more capability (multiple interrupts + extra pins), so looking again at the Feather M0.

Thanks in advance

@rvendrame You're welcome.
I suggest mounting the sensor on a 1/2" ball valve, that way you can isolate for cleaning/replacement etc., and if it means draining the tank to make the fitting it's a one time event as the valve isolates.