Yeah. Remember last winter when I used a tension rod and a doubled-over quilt to insulate these doors? It worked well; as noted the time or two that I pulled the curtain back and found frost on the inside of the door.
That would only cost us... let's see, carry the one... an arm and a leg. So the next alternative would be insulating drapes that we can at least pull shut to block the doorway on the coldest of nights. Kind of like the curtains my mother in law sewed from post-holiday sales tablecloths at Target for the dining room.