..I have a need to make a very thin extension ring for a Mamiya 645 - thinner than the thinnest commercially available. It needs only to be about 3-4mm thick to usefully shorten the min focus distance of the 45mm lens. Of course I want to do this for minimum cost. I'm thinking it may be possible to screw a spare lens mount to the back of the existing mount on the 45mm - using the existing screw holes as long as these lined up accurately enough and if I could source much longer screws so that they would pass through both mounts and into the lens body itself. Any idea where I would get such screws from? Of course auto aperture would be lost - but that's not important.