I just got a used Polaroid 545i film holder for my Shen Hao 4x5. The film holder easily slips onto the back like a regular two-sided film holder, but there is a slight problem. I disassembled the ground glass and back to discover that the small ridge on the Polaroid 545i film holder doesn't "snap" into place like the one on the regular two-sided film holder. As a result, the ridge pushes the top of the Polaroid 545i film holder away from the camera back by an amount equal to the thickness of the ridge itself. Essentially, the Polaroid 545i film holder doesn't stay flat once inserted, but sits at an angle with the top of the holder farther away from the back. I haven't exposed any film as I received the Polaroid 545i this morning. I'm considering filing down the ridge, but would like to avoid it if possible. Anyone with this problem and has found a working solution?