I blocked out the window with a couple of layers of the garden plastic sheet and velcro on the sides but attached to a dowel stick retained above and below with "L" hooks. For the two doors in my darkroom I used black out fabric as you mentioned. Also attached to dowels both top and bottom generously over-lapping the door frame but only attached with the "L" hooks on the top and easily removed and rolled up like a scroll when not in use. They have worked without problems for almost forty years.