Edit 8/21/15: THIS THING IS OBSOLETE! After testing I am proud to present the new Universal Magazine Holster:
http://www.thingiverse.com/thing:977519

It is far superior to this design!

Edit 6/18: I have successfully modeled and tested the clip for the new version! This might not sound like much but the clip is probably harder than the rest of it. This project is officially being processed!

Edit 6/5: I have been quite busy and the new version hasn't left the paper (better than just idea right?). It is next in the design queue however, so when I get some time I'll try to work on it. I hope to finish it by August of this year, but that is a loose date.

Edit 3/15: A new version is in the works and is expected to be completed in a month from now unless I get bogged down with other work. (I want to finish in time for next month's match)

Edit 2/12: This has been tested and the clip is extremely difficult to insert, although with enough filing it works. The spring is also very stiff, so a less stiff spring will be available soon. It does have very nice retention, however.

I have concluded that I could get away with much less plastic in the body (I'm not a ME by trade give me a break) so I think a major overhaul is in order :|

Print with less infill or wait, I'd recommend just printing with less infill.

I needed some decent mag pouches for competitions and I've seen other designs on Thingiverse, but non of them were implemented with a spring clip like other commercial designs do. This should hold anything shaped like a Glock magazine, and I tried to make it short enough that it could accomodate a Glock 19/23 size mag as well as the full size ones. It might fit a 26 size mag too.