Most important things in coop design
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Pull out cleaning tray lined with scrap linoleum is nice in addition to your doors, you can spray it off with a hose and bleach if it gets too nasty. Poop boards also keep poo in the bedding to a minimum if you don't use deep bedding method. Again line with something that can be sprayed off easily. If your coop isn't big enough to get your body into you want to be able to reach to the furthest point. If you need to get a bird for inspection or something, there is nothing more frustrating than when they are just out of reach. I can usually my small rake to corral them out tho. Other than that it sounds like you have it covered with ventilation and the next boxes.
You also will want to close off access to the next boxes until they are big enough to lay. So they don't think its a hangout spot and place to go poop.