If all you need is a stiff neck for the opening, then sew some thicker leather to the sides of the NECK, using the same seams that already exist. The two layers can be Pinched to open the bag to pour the shot( or ball) out. When you release the sides of the neck, it closes, to help keep the shot in the bag. The thickened neck will lean over, of course, from its own weight. If you make a neck long enough you can sew a keeper loop down low on the neck, so that the neck is folded over, and shoved under the leather keeper to save the shot from falling out of the bag.
The same idea makes a great ball bag, that delivers lead balls one at a time.