drafts. well i wouldn't worry about making it with light weight batting. you want a bit of heaviness to it. i would make it to fit straight across, no bunching up at all. make it a bit larger than the door and hang it on a dowel rod above the door and curtain rod. or remove the regular curtain and rod at night so the quilt will fit flush against the door at night.
good idea. it can be done and should help. just a pain to put it up every night. good luck.