I found no satisfactory mag pouches here - so I decided to create my own. Hereby I present the Mag Pouch Mk2 - designed, created and tested in the homeland of Glock.

It features variable retention, an exchangeable click-on buckle and a sleek design. Any Glock double stack magazine should fit - even though, I can not confirm it for the G21 - some feedback would be much appreciated.

Currently I only designed a closed loop buckle but am thinking about developing one that is similar to the Bravo Concealment IWB buckle.

Required for one mag pouch is one M3x16 screw and a fitting nut. I like to use a locking nut so it doesn't come off when it is not under tension.

Both the mag pouch and the buckle need support for printing correctly. For the mag pouch body itself I recommend a high support angle so that only the gap at the cut out retention piece and the latch for the buckle is filled. I used a zig-zag support pattern without a roof or floor, which is easy to remove.
For the buckle you need more support. I recommend using a lower angle, so that the dovetail is supported too. In that case you need a roof for the support to get good print quality at the dovetail.
It might work without supporting the dovetail (with some powerful bed adhesion) but I did not test it.

I will test the support situation some more but in the meantime I have attached some gcode files with settings out of Cura that worked for me (and were outlined above).

I used PLA but assume you will get better results with PET-G. It is more flexible and will do better with the buckle assembly. In my experience the PLA will lose its tension and straighten out at the angled buckle latch.

I printed it at 100% infill, since it makes it more durable and hardly makes a difference in print time or material use.

I will soon design a disconnect tool that makes taking off the buckle much easier.