A Simple Coop
This is a simple design and fairly similar to 2 coops that I built in other backyards. For both of those, I used a 4' by 8' sheet of plywood for a roof. Here, I think I'd use a full sheet as a floor with treated 4" by 4"'s as joists. Perhaps, a cedar board roof would be appropriate. The walls could be 4' high but access would be greatly improved by making them, at least, 5' tall.
I used windows as a double (French) door on 1 coop. That worked well but probably wasn't as attractive as installing a separate door & window. Vents are along one wall in this design. The sketch isn't perfect but it may be of some help.
Since my current "Bird House" was made to slide into the bed of a pickup, that coop is much more complex. It kind of gripes me that I built it that way and have never moved it. Maybe, I should load it on the truck and take the hens for a ride just for fun !