I turned my Hasselblad into a pinhole camera using a piece of mat board (cut to fit within the lens mount) with cutout to hold a piece of foil with the pinhole. To prevent light leaks and make the fit tighter, there is some electrical tape wrapped around the edge. It works pretty well and I haven't had any problems with light leaks. Viewing is difficult, but if the scene is very bright you can see a very faint image on the ground glass.