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.
I am very much an enthusiastic amateur when it comes to designing. My designs tend to have two things in common
1) They serve a purpose. I like to design functional objects which have a real-world use. 3D printing for art is fine, but it's its real-world application which will take the technology forward
2) Wherever possible I design to be easy to print and try to eschew supports or complex printing; modifying the design where necessary and possible to achieve this.