Hah expensive.
Cheap solution. Those old school CF surplus SMG mag pouches.
Hold 3 MP5 mags each perfectly, an they are at very most, $5 each.
They even had a flap inside them that if ya mag is sittin low or tight in the pouch, pull it an the mag comes flying up and easy for you to extract.
Mind the black and white picture, but here's how they look with cadpat and an MP5K.
If y a use those pouches, I advise having a dumb pouch on your belt as well so you can just drop your empty mags into there and keep moving.