This is a birdbox which I've designed to specifically be easy to print in one go with no supports needed. All of the overhangs are supported directly within the design so that the box can be printed in one go.

It has a 28mm hole which is suitable for birds up to Great Tit size (as far as I can see).

I have incorporated a single screw (or nail) slot mounting on the rear to enable it to be very easily mounted to a tree or fence.

Ideally print in something reasonably weather proof, but I don't suppose it'll matter too much.