You could try using a matt-board as a substitute easel and a rectangle of board as a mask, a few mm smaller than the printed area, to "fake" the black border. Put the rectangle on top of the paper in one corner, leaving the 'gap' on two sides only. Expose for black, move the card to the diagonally opposite corner and expose again - that way you get a symmetrical black border, but you could mis-align the card deliberately for some asymmetry if preferred.
Doing it like this means you probably have a very even black line, and of course no sprocket holes. Sprocket holes can be easily added with a second exposure however, see my post
in Taylan's thread in the darkroom section. If one is careful with alignment it is also possible to reverse a piece of exposed film leader on the interneg and simplify the described process further.