For my broodies, I use a wooden rabbit nesting box. It's about 10" x 18" and is my preferred, go-to nesting box when it comes to hatching. It works great for the broodies and gives frontal space for the chicks to go to without leaving the nest. The only thing with this nest is I like to make a ramp with shavings around the front of it because it is a little high.Its a 12in x 12in i think. But ill have to go measure again. She can keep them under her ok but its just when she tries to turn them she has issues. And i will be making the boxes bigger. She is a full sized Buff Orpington.
Here it is:
(It's a little dirty and hasn't been used in a couple of years. It's last use was holding screws after a long job. )