I've never done this before...so Im taking a stab in the dark here....But what if you were to cut out a piece of material that would be your image size, overlayed it onto your photo paper, and preflashed the paper until you achieve dmax for your border - then removed the cardboard, and made your photo exposure ? This would give you sharp lines inconsequentially of your border around your negative carrier, etc.
Beyond that, you could file your negative carrier, but I don't think the lines would be as sharp, and cropping, etc would be an issue.

Maybe ?