Hope you're sitting down.................
I make mine from empty, plastic coffee cans.
I punch drain holes in the bottom. I punch vent holes around the circumference an inch or so below the top. I cut the proper size entrance hole. I spray paint the lid with light gray primer. I take off the lid.............drywall screw the can to a tree or post and put the lid on. Often, I'll place a 1/2 pound - 1 pound, flat rock on top to discourage monkey business from prying predators.
The can on the pole in my front yard has reared upwards of 15 - 20 broods.