File at an angle of 10 deg. Borders will not go black because the blacks in your print are not max paper black.
They simply look that way.
Glass is better. I filed out the top to take AN glass from a glass slide mount. You can not cut it as it is too fragile.
This is a long slow file operation with lots of trial fits to keep square and not go over. Do it well and it will become your favorite carrier.
Omega 4x5 glass carriers have two locating pins where strip film would properly extend. The work arounds are masks cut at an angle or have a shop relocate the pins.
Neither is a nice solution. On older Omegas, the carrier will not rotate to compensate for the crooked neg.