Works great in a JSTA pouch if you cut it in half.
If you're willing to cut it in half, it fits fine in the pocket of the JSTA pouch with steel and plastic MP5 mags. You might be able to fit 3 slots in if you're a surgeon since there's a lot of excess in the outside seams, but I wasn't that bold. The JSTA fits 2 MP5 slots safely, with a little room for a pistol mag or flashlight or glow sticks or anything relatively mall or narrow down the side. You CAN stack MP5 mags quad style 2x2 in the JSTA, but it eats into the internal pocket storage. It holds the mags quite strongly. If easy insertion matters to you, it does not insert easily in this configuration. I used this for spare mag storage on a BoB, not a plate carrier or rig, so that doesn't really matter for my purposes.