I cut masks out of thin sheet aluminum flashing from the roof section of the hardware store. I just lay them on the glass and use a piece of gaffers tape to hold them in place. Cost about a buck. I used my Rototrim to cut the flashing to size. I was able to cut the holes with a sharp knife. Works fine. If I could afford it, I would buy one holder for each format and use red litho tape to mask the negs off. That would be the best scenario. I also use the flashing to block the light leaks that the Saunders has. The aluminum doesn't get hot (although it has sharp edges) and it just sits right in the slots on the head around the heat sink where all the stray light comes from. Works great.
I never liked cropping masks. They do cut out light, but they leave a very fuzzy edge that can screw up the edge of the image if you are not careful.
I think I might have tried matboard to make the masks, but they didn't clear the housing when inserting the carrier. I also thought I would be a cheapskate and give an Omega glass carrier a try but it was too thick to clear the housing as well. The space for the neg carrier is pretty thin on the Saunders.
If I could go back in time I would smack upside the head the guy who decided to make the 4x5 glass carrier too small to print a 4x5 neg....
