I use the Victorinox Swiss Tool sheath/belt pouch as a discrete and inexpensive spare mag holder. Fits single stack or slightly staggered 9mm mags with 7 or 8 round capacity perfectly (tested with S&W 9mm Shield and Keltec PF9 mags). For shorter mags, you can make a cardboard spacer to slip in the bottom to raise the mag. Too narrow to fit full size higher capacity true double stack mags though.

https://www.amazon.com/Victorinox-SwissT...ctorinox+sheath