This has nothing to do with bows. But here's a half dozzen blue bird boxs I built out of PVC pipe. I have to paint them and add a perch yet. Blue birds nest in old hollow fence posts. I just sucure them to fence posts. You have to put them 150 feet apart. Blue bird nesting territorys are use'lly around a 100 feet. I put mine 150 feet apart.
Make them a longer and put them up 12 to 30 feet in any hard woods and you have fly squirrel dens. 3 To 4 feet.
I also make squirrel dens useing 8 and 10 inch pipe. But remember any squirrel dens has to have bigger holesalso you have to have a branch inside for them to climb out on. I add a strip of 1/4" mesh wire couple inchs wide as long as the pipe. Just bolt it to the pipe for them to limb out on.
Most hard woods can suport a lot more squirrels thans there, there's just not enough den trees. My granddady always put lots of these squirrel dens around our squirrel woods. Put them where you want them so there more huntable. We always had tons of squirrels to kill growing up.
If you make the hole ALOT bigger and put them up higher in the trees. Lots of times owls take them over. Use the 10 ,12 inch pipe for owls not two tall 2 feet. You also have to had the wire for the owls to limb up to the hole so the can get out.
Put them a good 150 ,200 yards apart so 1 owl down own more than 1 roost.
I always add a lot of all to the farms I hunt,least and the ones I manage (2 right now)