I designed this birdhouse for tree swallows and violet-green swallows.  However, I can not confirm this has been occupied by either.  The small hole opposite to the entrance hole is to attach a rubber-coated magnet with a threaded post.  You can find suitable magnets for sale online.  Equipped with such a magnet, this birdhouse can be clicked onto a steel pole.  I rendered this birdhouse in black PETG, and then I sanded it lightly, and then I spray painted it with white primer suitable for plastic.  In full California sunshine, the painted birdhouse did not become too hot inside, given the ventilation holes at the top and bottom.