I have used just a single, community nestbox for nearly all of my backyard flocks. The entrance allows one hen at a time but 3 can squeeze into the nest if they are desperate.
The nest entrance in my current coop is square and a 10" pot happens to fit perfectly. Plunk.
If I have a sleeper and I'm a little late getting in there with the "door," she won't get tucked in! Unceremoniously drug out by her head will be more like it. Thought that might be a cozy bed? Wrong. Lucky ya don't get slapped.
Cat, the original nest box stayed the same, the only thing my husband did was cut a couple inches from the back to allow the egg to drop. He placed a thin strip of wood across the opposite end and layed a piece of plyboard on top to create an angle floor.
He then built a drawer and did the same thing as the nest box to create a sloping floor. Note, both the drawer and nest box have 2 floors. The bottom one and then the piece of wood that is on top sloping down because it's being lifted from the top by a thin piece of wood underneath. Hope this makes sense. The drawer slides all the way out. Both are lined with a piece of carpet.
Haha Catjac. The box had only a few minor modifications from the inside. The good thing was that he didn't have to change the outside at all. I initially thought he was going to extend the back of it which would prevent be from fitting back there. That ugly piece of wood is a temporary lock. I just turn it. Don't know how effective that is against predators. I'm not crazy about it but it works.