I am looking at image like this. Notice the inner thin edgy border in black.
Given a film negative and a scanner, how can we make such border?
What I do is to let the scanner (or its SW: I use Vuescan) find the frame in the neg and then just slightly increase the frame size include some of the rebate (the unexposed edge of the negative). That's not hard to do.
(When printing negs I gave up on the filed-out-neg-holder thing and now use glass which are a pain to get the film positioned in but otherwise much better.)