Simple plunger-style speed loader that is a lot more portable than the usual tabletop slider-style loaders.  Requires no additional parts.  Set the loader on top of the mag, slide 15 round into the loader, and push them into the mag with the plunger.

Print the two parts as provided.  Loader needs supports (regular, on buildplate only).  Recommend ABS/ASA or PETG.  Recommend 4+ walls and 100% infill.