What if, A piece of this flat, pliable, magnetic strip - say an inch wide was sewn along the leading edge of your dark cloth . Now it would require two pieces hanging vertically along the edge that hangs down near your camera bed and straddling your ribbed area. In other words put a piece of ribbing in the middle and on each side a little further out two strips of magnetic material. Now since field cameras are built out of wood and non-ferrous metal there would be no worry of the cloth adhereing to spots that you didn't want it to. Then you would simply put two thin sheet metal strips on the ends of the light trap material that would adhere to the magnet that is sewn into the dark cloth. This would be so much easier to explain if I could just draw you a picture....lol