PKM, I made my own for my Saunders 4550. I started with the glassless carrier, milled out, and installed my own glass. An easier solution if you don't need the metal frame, is to simply sandwich the negative between two pieces of glass. Blacken the edges of the glass, and mask it with black construction paper. Plenty of people make masking carriers that way (Lynn Radeka, Alan Ross etc) and it works fine. It's actually a useful method because you can easily swap the top glass for different requirements (diffusing plexi etc).