Mag pouch.

[Wardog61]

The USGI version of the MOLLE frag pouch may work. If the height is correct, it should hold at least two, maybe three of the 15 round mags. The 40mm grenade pouches may work as well. Depending on the style, they might hold up to two mags per cell.
I'll check both when I get home from work tonight and let you know if they work.
In the meantime, take a look at this pouch. I have a couple of these on one of my vests to carry a cellphone and some other non-critical items. It may work for m1 carbine mags too.

Tactical Advantage Product: Tactical Tailor Flashbang / Small Utility Pouch