by grinding out the aperture on my negative holders for 120 film, for 135 I got a factory made holder (Beseler 8302) that gives me the same result. Make sure there are no sharp ridges left that can damage the negative and paint the edge black so there are no glare from the shiny surface. Of course this will only give full frame black borders, cropping are done in camera;) If you need to crop you can either reshoot or make the border in the easel:)
