just a generic one from amazon, i find it extremely useful not just for mags but for grenades/bipod/armbands/whatever else i need to carry but can't be bothered having a dedicated pouch for.
plus dropping mags in the dump pouch rather than the mag pouch is an easy way to differentiate which mags are full and which are empty.
I have a really cheap viper one. Works perfectly for what I need. Take mag out, dump in pouch, put new mag in. Then if I’m in a quiet patch, remove mag and put back into pouch.
Depends on on your play style, I play fast so saving a few seconds is always good for me!
When I'm using midcaps I dump the empties. I don't keep other crap in it, I have utility pouches for stuff I need to carry and a bag in the safe zone for stuff I don't.