Check out this 3D-printable birdhouse I designed! To attract different species, I've included three opening options: a 35mm front for nuthatches and house sparrows, 

and an adaptable front with two swappable rings—25mm for coal, marsh, and blue tits  and 28mm  for great tits and tree sparrows

I intentionally left out a perch to prevent predators from harassing the nest, and added drainage holes in the base just in case. 

While you could glue it shut, I recommend using screws so you can clean it out at the end of the season.

Pro tip: Use a heat-resistant filament like PETG or ASA; standard PLA will warp in direct sunlight!