I have had really bad luck with elastic loops for shotgun rounds. Poorly designed loops are too loose from the beginning, and rounds work their way out when you walk, run, jump. Better designed loops wear out quickly, with the same result.
I don't know if you have access to a good sewing machine or not, and whether your unit would let you modify your vest. If possible, you might take a look at the shotgun speedloaders on the vest at the link I'm attaching. These speedloads do not lose ammo. Also, they feed downward, so you can reload with one hand. Each loader holds 4 rounds, and when you remove a round from the top, gravity moves the next round into place.
I'm not sure the best way to attach something like this to webgear/vests---that would be for a good guy with a sewing machine to figure out. But, I do know that if my main gun was going to be a shotgun, I would damn sure have a way to speedload it.
I have also used old SAW drum pouches to hold shotgun ammo. They actually work fairly well when affixed to your vest, hold quite a few rounds, easy to get into, easy to close, and don't lose any ammo. Trap and skeet shooters often carry a similar pouch.
Sorry to send you to a "cabelas" link, but it was the 1st place I could think of that would have the speedloader design I was thinking about.
http://www.cabelas.com/product/Cloth...3Bcat104058180