...Take a piece of stiff fabric (lighter than seatbelt, heavier than tarp, I used a waist strap from a pattern '82 rucksack) 2" wide by 16” long. Mark 7” in from one end, fold double.
Starting from the fold, sew joints across the strip sized so that the ammo of your choice is a snug fit in the area between the lines. This takes some experimenting.
At the >2" of single layer end, cut a button slot and finish it by sewing all around.....
I cut up one of those elastic shell holders and sewed it inside my jacket pockets.