So I got one of these things from asda a while back.

I opened it up and there was plenty of space for 'things' inside. So I connected a 3.3v regulator to the USB circuit to power a pro mini. On that, I am running the example humidity sensor sketch with repeater mode enabled. Connected to that is a DHT22. I cut a hole in the side for that to poke out of. I also managed to get the pro mini and NRF quite well stuck together. Compact is probably the word i'm after:

I tried to keep this away from the high voltage side as best I could. The gap I managed to stuff the pro mini + NRF into is a insulated area, so perfect! I am quite pleased with it.

It did just occur to me though.... the DHT could pick up heat from the USB circuit inside. I'm not sure if it runs warm when in use. I suppose I will find out.

@hugows C4 goes over the ground and VCC of the NRF radio. I'm not sure about but C5, from some pictures I have seen I think it goes in the same place as C4 but I may be wrong. I have left that out without an issue.

OP - this board is great! it take the pain out of soldering the radio to a pro mini / nano. I really like these.

C4 in your picture looks correct to me. You could put C4 up on the edge under where the radio would be as well. You have VCC up on the edge. I think the pins adjacent to that are ground too. EDIT: I think I replied before you edited your picture :-)

You can solder the components underneath if you need to yes. I don't see why that would be an issue.