If you want to carry a 20 round magazine, you can make one yourself. Buy a Bianchi UM84 magazine pouch for double column pistol magazines (Beretta 92, BHP etc.). Remove the belt clip from the back. Then remove the plastic liner from the nylon pouch. Go to the hardware, sporting goods, home supply store and buy some "Velstrap". It's just 1" velcro, hook on one side loop on the other with a delrin buckle on one end. Thread it through the belt loops on the back of the pouch, thread it through the sling cuts in your collapsible stock and tighten it up. The velcro allows you to cinch it up tight.
The modification to the Bianchi pouch for holding a 20 round AR magazine was posted to the AR15 list a couple of years ago. I made my first stock pouch out of one using a couple of straps I had laying around that were for diver's depth gauges and compasses. I have since found the Velstrap available in various lengths.
If you don't want to make your own, I think The Wilderness
http://www.thewilderness.com makes one that is used with their Giles sling.
Jeff