I use 1 by 12 rough cut pine. This way I do not need a ladder on the inside. The hole should be 4inches long by 3 inches high, like a football shape, anything else invites predators. Trust me, they fit perfectly through. The front is 22inches and the back is 24 inches. I use liquid nails and wood screws. The back I make larger or taller ,so I can attach my lid and hinge and if you are sticking them to a tree it helps. Be sure to put a locking device on the lid. Fill with 4 inches of wood shaving bedding, not wood chips. The hole should be as close to the top or lid as possible without compromising the integrity of the box strength. 3 or 4 inches. If on poles be sure to predator proof the pole. I have had to double plate my holes over the years because the coons try to chew there way in. That is just another piece of 1 by with a hole , usually 12" long by 10" wide and srew it in with 1 1/2 srews, so it is double thick at the hole. Do this on the outside of the box, not the inside. You will have to clean them annually, I do mine in late Feb, because the ones that are over water are easy, because the water is usually still iced up. Be ready for squirrels and screetch owls during cleaning. ENJOY:cheers::thumbsup: