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Exactly. Also consider if you will have more hens laying in the future. I raise replacement layers so I often have more hens laying than my base laying flock. I have one hen that will spend about three hours on the nest every day, whether she lays or not, and she will not allow any other hen to climb in with her. I think at some time in the future, for whatever reason, you will be glad to have an extra nest.
The minimum size I'd build is 12" x 12". I made mine 16" x 16" with the thought that they will often climb in there together and that will give them room to both lay comfortably. There have been pictures on this site of a hen laying on top of another hen when the nest is tight for space.
Don't forget to allow sufficient height. I find that I need a lip about 5" high across the front to keep mine from scratching out the nesting material, fake eggs, or real eggs that may already be there. I had a nest that only had a 6" gap over the lip for them to climb in and it worked, but I like a bit more, say 8", to make it a little easier, especially if the nest is elevated where they have to jump a bit to get in.
Plenty of people successfully use buckets, cat litter boxes, milk crates and a lot of other things as nests. There are no hard and fast rules about this. Some people set a milk crate in a corner. Some of us build elaborate contraptions and call them nests. The chickens usually use whatever we offer and sometimes they do their own thing. Good luck with however you decide.