Here’s a nice video showing a man and his son building a coop out of a lot of re-purposed materials. I can’t tell from the video, but my only suggestion would be to make sure the whole thing is sealed. It looks as though the back wall has an opening under the roof, which could allow raccoons in.
Neil is in our yard right now trying to figure out how to secure our coop to keep marauding raccoon claws out.