I would try using 4-8 ply mat board, maybe stained black at least on the borders. I would cut a window (just like for mounting) with a beveled edge. I would make the dimensions of the inner cutout just a tad smaller than your print size to use as the mask. I would tape a small piece of scrap board to the top of the mask so that it is easy to lift. Then I would expose the print in my 4-blade easel, then place the mask on top of the paper, then open the blades of the easel, then open the lens for a brighter light, dial in maximum magenta for maximum black, and expose the black border. Should give a clean line around the print. Obviously, this would work best when printing numerous prints of the same size.