We have built a couple from wood stripped frompallets. Much warmer than the plastic one we bought and no condensation unlike the plastic.
The first doesn't have external nest boxes and the floor was pallet wood. It was to bring youngsters on so no problems re nest boxes but the muck gets trapped in the grooves between the wood.
The second one I love, warm, good external nest boxes and a solid ply sheet for the floor. So much easier to clean. There are natural small gaps where each wood meet but doesn't pose a problem in bad weather.
The roof is onduline to stop mites.
Wish I could upload piccies but not very iPad savvy
?
There are a good few designs on line to help. We are going to build a better one just to tweet access for cleaning, and sell the plastic one.