What should I make a curtain out of to make a room divider for the air conditioner?

We don't have central air and we live in an old house so we don't have a lot of doors between rooms and the door ways are strange small sizes so we can't put doors in without custom making something. I decided that I would make curtains but should I just make thermal curtains or should I line them with a thin plastic?? Any suggestion or if you know of anything pre-made or what type of plastic.... All appreciated.