In reply to Timmd:
Whatever you fit it'll need to fit closely especially at the top to avoid drafts and thermo-syphoning.
Any heavy curtain that sweeps the floor and is appropriately detailed at the top and sides (velcro or magnets maybe to keep wall/door-frame contact) will do. If you're feeling crafty and/or it's especially cold one side or the other you could sew in a foiled mylar emergency blanket and a layer of thinsulate between the curtain and the lining. They probably won't drape so well unless they were very heavy to begin with but they'll keep a fair bit more heat in.
Quick and dirty you could hang a Tesco value winter weight double duvet for less than the curtain hooks would cost you!
I'd be tempted to put a door back in
jk