If you are using a box camera you want to cover the window, but you only need a flap of electrical tape. With the older Kodak type cameras the problem isn't light getting through the window and striking the back of the film, the problem is that light gets in and bounces off the film and reflects off other parts and finds it's way around the film and to the front of it. In my Kodak cameras I have added a foam film backer, and bent the original springs (those long rails that hold the film to the frame) and I still needed a window flap.