I used a Dremel, then a smaller file in the one I filed out. Depending on the enlarger and size neg, there are full frame holders out there, too. I have one for 35mm for my Beseler 23CIII, but had to make one for 6x6. Using a hand file will take awhile, but is less likely to take off too much. The trick is to keep checking so you don't make it lopsided and then way too big.