After besting the BBEG, they discover a set of plans for 3d printable minis. You could print/order them printed and have them ready to hand out after the campaign, or if its a long-distance game, distribute the 3d files so the players can print them themselves.

Bam, they’re given physical form that can persist beyond just memory. Monkeys paw-esque twist: they can now be used for other D&D campaigns with more monsters and evil assholes.