Filed-out carriers used to be a punishment for me. If the teacher thought you were composing on the enlarger too much instead of in-camera, he took your carrier and gave you a filed-out one. You would have to do the rest of that project using the filed-out aperture and showing sprocket holes on both sides.
Filing out an aperture is simple in theory but can be difficult in practice.
I have filed many apertures on cinema projectors. It's the same idea as filing a negative carrier's aperture: Just do it.
Take an old negative and use that as a guide to know how far you want to file.
Project an image. File a little bit. Project again. Repeat until the edges are where you want them to be.
Work slowly. Be careful.
Use a good quality file. Nicholson, et. al. You'll pay for good tool, many times over, buying cheap tools.
I used a 4" Pillar file. (They are narrower and the edges are parallel.) #4 Swiss cut.
Pillar files also have "safe" edges that have no teeth so you can file square corners without blowing them out.
When you get the edges almost where you want them, take the file and bevel them back. That will give you a clean-looking edge when the image focuses on the screen/easel.
Before you finish, take a fine emery board (manicure file) and polish the edges perfectly smooth.
Okay, so filing projector apertures is a lot more critical than enlargers but the principle is just the same.
Measure twice. Cut once.