Now I'm able to screw panels together using any side:
I connected solar panels together (parallel connection, but each panel in series with it's own shottky diode). 11 panels on north side (direct sunlight from about 06:30 up to 09:00) and 5 panels on south side (direct sunlight from about 12:00 up to 19:00). Panels located on window-sill outside of house.
On south side I'm using 6V AGM VRLA acid battery (as far as I remember 4.5 Ah), on north side battery is the same, but battery capacity is 1.3 Ah.
At first I connected panels directly to batteries (panels open-circuit volage is about 6.8-7.5 volts), but I got weird results. When it's sunny and the sky is clear batteries are charging very slowly and then it's cloudy - batteries are charging faster. Scattering of light by clouds?
Then I connected solar panels to step up regulators (mt3608 as far as I remember, output set to 7.0 volts), now I'm getting very good results. Both batteries are charging almost all the day.
Graphs showing step up output voltage (connected directly to battery):
Again, then weather is cloudy batteries are charning a little better.
Also it seems that I have connected too many batteries. It's necessary to try only 1-2 batteries on each side.
Now I'm using simple voltage reporting sketch (5V arduino pro mini, report voltage and smartSleep() every 10 seconds), battery connected directly to RAW pin of arduino. Nothing is changed on hardware and software sides except I removed power led. NRF24L01 using it's own AMS1117-3.3 regulator (consumes a lot of current, about 2mA) connected directly to battery.
And it works fine, whatever the calibration error is, it's consistent.
So in reality, the finished part size might not be exactly that, but whatever the difference is, it is propagated to both parts. This probably wouldn't be true if I printed the parts separately in different printers.
Anyway, we're talking about tenths of millimeters.
@gohan Well, in my case, it is going to replace my thermostat, so I already have the 5 wire cable coming up from the basement for that which takes care of the hole in the wall. Since the main controller for the thermostat is in the basement, all I should need at the tablet side is a temp sensor. I will use the 5 wire cable to connect my 5 volt charger and probably a DS18B20 temp sensor to read the temp. In the basement, I will create an arduino node to read the temp and send it to the thermostat controller over my RS485 network. All the other thermostat controls will be handled by the tablet.
It was not so hard finding one. I got it from the Pi Hut because now and then they get a shipment and via their newsletter the word is spread. Actually, they seem to have it in stock right now. Just pick the "Just the Pi Zero" option to avoid all their extras