I like your idea of making it to the dimension of the door, no gathering unless open. Back in the 70's there was a mag that had a one fabric quilt for the window cover and they took the lower corner and folded it back on itself and attached with a button, unbutton to close. Friend of mine had a scrap quilt of many colors, with a thin batting and when she hung it over the window it looked like stained glass with the light shining through.