A small part of my team fortress RED Sniper cosplay that I had an absolute ball with.
The pouch is a small detail worn on the right hip. After scrutinising source images carefully, its apparent that its not hanging from the shoulder, nor does it go completely around the waist or thigh. To sit the way it does, it needs to dangle from the belt.
I made mine using these dimensions (in inches), which should be of adequate size for most snipers. A rotating leather punch is tedious, but not as tedious as shoving a needle through leather with brute force so many times. That said, I've probably been a bit masochistic here by using leather when a dense, rigid cloth might work just as well. The resulting pouch is perfectly useable outside of cosplay though, and makes a handy place to store my handphone, digital camera and wallet during. So I didn't mind going all anal and making it well.
The brown belt that got turned into the strap is sewn onto the back of the puch just under the level of the opening, and is itself modified as at top right. Ideally, I found it a good estimate to let it hang just about level with my crotch.