I went ahead and built new nest boxes this afternoon, and installed them. They're made of scraps of plywood I had left over from the walls. Each opening is 14x14x14 and it fits perfectly under the roosts. I'm really happy with how this turned out:
Carrying the chickens from their old place to the new one!!
Inspecting the new roost:
and, of course, most of the chickens hid under the roost area. I may have to close that off to prevent them from going under there.
They are curious and uncertain but by tomorrow they'll know this is home.
Here's the timer. Sorry for no plate being on the power yet. Nobody is allowed to touch this except me. Next time I'm in town I promise I'll buy a faceplate.