Making a print with non-original border effects came up last year in another forum. Here is my attempt at manually recreating the design of the digitally-reproduced example at the top of the linked thread. (That image in the link was made digitally as the agency digitised all their files years ago, and the original neg is rather valuable!).
To show a little of the edges of the neg around a print, the simplest way would usually be to use a glass neg-carrier, while using the carrier's masking arms to limit the amount of extraneous light to just the narrow strip you need. This method is shown in the 'straight' version of the print I made in the linked thread, though showing pretty much the whole neg rather than a narrow strip.
To make a simple, evenly sized, black border around any format of print, decide your cropping and then cut a piece of black card (or 3mm foamcore) the same size as the exposed area of the paper. Decide how wide you want your thin black keyline and trim the card by that amount on two edges. Make two fogging exposures, one each with the card pushed top-left and bottom-right to produce the black key lines symmetrically.
Using a glass neg-carrier, the sky is the limit to the strange edge-changing items you can put in there with the neg, if you didn't want to make your chosen effect with multiple exposures.