I used a rabbit hutch to build my coop/tractor and a run off of it. I have a hole cut in the back wall of the coop with a board for them to get in and out and was wondering if I needed a door on it? It's in my yard with 6 ft fence so the only animals I think could get in are birds or moles. I'm am mostly concerned about them being able to stay warm in the winter time if the door is open, I did put a "shutter" on the front part that was all wire to be able to close up and protect from the elements. Would a cat door work? They go in on their own at night from the first night they were out.