Posted: 29/03/2014 at 18:21
I worked out my own design based on RSPB specs, which called for an entrance hole 150mm above the floor and a 125 x 100mm floor, if I remember correctly. It uses a 1'x8' plank. It's mostly the same, but a bit taller. I gave it a 45-degree roof slope to shed Yorkshire rain, and attached the roof to the side walls using lengths of 25mm square wood screwed to both roof and wall from the inside. I used the same to attach front to sides, meaning there are no visible screws up to that point. The back wall and back edge of the roof are cut at 45 degrees so the roof fits on the wall top. The sides are screwed to the back wall directly. On the house, the ivy hides those screws.
The floor is slightly smaller than the internal area of the box for drainage (again, Yorkshire rain). It's got two rails under it that aren't square, and they're screwed at slight upward angles into the side walls. To clean it, you remove those screws and drop the floor out.
Having made it, I had a single piece about 5cm x 50cm left over. This I drilled at both ends, took up a ladder and used as a template to get the screw-holes in the right places on the wall. Having done that, I screwed it to the back of the bird-box. This way I didn't have to hold the weight of the whole thing while marking the wall.
A note on mounting: if you mount them on a wall, fine. If you mount them on a tree, the screw threads are holding onto the wood inside the tree, the heads of the screws are holding the box and the living outer layer of the tree trunk *between* those places is growing and producing more layers of wood. This will push the box outwards until it falls off, leaving the screws completely buried in the tree. One of mine fell ten feet onto clay and made a square hole in it. :)