I have been making enlarged negatives for years using x-ray duplicating film. I use Kodak X-Omat 2 film with Kodak GBX chemistry (there are probably other brands). It is as easy as making a print. Reverse the original negative -- have the emulsion up in the negative carrier. The duplicating film is a reversal film. It is very slow and remember more exposure yields a "lighter" dup and thus a "darker" print from the enlarged negative.