Has been asked before, but are there any commercially available products or 3D-printable inserts to convert STANAGS or USGI 30rd magazines specifically to stack 9mm?

All the conversion products out there (MEAN Arms' EndoMag, CMMG, etc.) are all for PMAGs. Is there just an inherent issue with the feed lip geometry of STANAGs for this purpose?

More difficult to take apart for some, and I've had stanag mags that took magpul followers easily, and some that I would have to sand the follower for a half hour to get it to fit properly, so tolerance stacking issues come into play vs a molded mag. I would be interested to see a comparison to pmag sales vs stanag alum mag sales these days, lotta OEM and gov contracts have gone to pmag.

Valid grounds, especially in relation to functionality; guess I was wondering because I really like the aesthetic of a USGI. Building a GWOT clone in 9mm, and figured the best way to get close to it would be to have USGIs to go along with, not PMAGs...

Have you tried to fit an existing cmmg/mean kit to a stanag for shits and giggles? May be able to make it work.

When the Endomag inserts first came out all kinds of attempts were made to adapt them to metal mags.  None were very successful.