Use a fine tooth file, or some sandpaper, maybe 180 or 220 grit and work slowly. Run the file or sandpaper along the full length of each edge, test it often with a neg you don't care about. Once you get to where you like it, there will likely be a small burr on each edge of the opening, remove this with a couple of light strokes of the sandpaper or file. Or better yet, finish things off with some 400 grit paper. Then blacken the edges with a sharpy or some flat black. Do both sides of the carrier at the same time.
If you use the sand paper wrap it around something stiff like small block of wood, anything with clean square edges and fairly stiff, or anything at all besides your fingers.
If you're just trying to get to the edge of the camera's frame you won't need to take off much material. But, note that every camera will be slightly different, so a neg from some other camera may still overlap, or else leave a bigger clear border.
Most of my carriers were filed by the previous owner, I've found it's close to impossible to get the neg exactly centered in the window so that the gap of clear film is even all the way around. I don't print with the back border anyway so I always masking off a very small portion of the negatives edge, either in the carrier or at the easel, or both.
If you work carefully it will be hard to truly ruin the carrier, but it will be hard to maintain a perfectly straight edge and perfectly square corners, but getting "close enough" won't be so difficult. If you mask at the easel to the very edge of the negative, any inacccuracies in the carrier window won't really matter.
Last edited by bdial; 02-14-2012 at 12:42 PM. Click to view previous post history.