Fiber or wire? Just peeked at it. The sizes are so small, I think you would be forced to go wire. I would lay one in the mold and clamp it in my vise. After that, if you need more adjustment, you could go two ways. Take a small hobby triangular file and remove just a bit of material along the impression the guard left with the edge of the file. Do this on both sides, of course. You could probably clamp a metal ruler along the impression and score it with a carpet knife. Let the edge of the ruler be your guide. Then dress it with a fine file.