I use a stock mounted Eagle pouch:
http://warrior-gear.com/store/pc/vie...&idcategory=70
and hold a second mag in there (mine take 24 rounds each), as well as I keep a magnet to hold my adjustment allen keys and various length small hop up set screws (incase I need them), maybe a small bag of BBs too (I have the CA M24, the space in front of the trigger guard has a metal door, I can keep a bag of 100 rounds in there easy.)