This is a remix of the AR10 PMAG Spring Base from 7.treetrav sized for AR15 magazines. It is a snug fit, but it pops into place during loading.

Printables won't let me properly link to the original design for some reason, so here's the link to it:

And, if you want a follower (available in several calibers) that will match this design, take a look at the various models from jmynheir: