I'm no home decorator, but maybe you could try cutting interior matching heavy cloth in strips (say 3 inches wide) and stapling them to a fastened piece of wood above the door (like a post on post fence). If you can sew, add small weights to the bottom to keep them in place. If you can't sew, go to Home Depot (or like) find liquid rubber, and dip the ends of the strips in the rubber about an inch. The rubber is heavy once hard and meant to protect tool handles and such. It comes in many colors. The finished product would look similar to the clear plastic flaps that help insulate commercial coolers in restaurants and grocery stores.
As long as you slightly (1/2 inch) overlap the strips, it should provide a good barrier for air, but also be easy to get in an out of. Kind of like the beads used in the 60s and 70s.
