I am designing a new coop for my chickens and was wondering if I could install a "pet door" for them to use ? In Michigan it get's pretty cold in the winter and I would like to have a cover on the door they use to go in and out, without having to shut it every night. I have a cat sized door and was wondering if that would work, or covering the opening with a heavy plastic, like is used for big walk in freezers? Any Ideas??