"An elastic stock sleeve with ammo loops" - I dig mine but it's starting to slip a bit, like an old sweat sock that doesn't stay up like it used to. I think I might like a velcro one, or one that laces up if they even make those...
You may be able to wrap a couple of velcro straps around it, but if the shell loops stretch, it's not worth it. I would like a lace-up model too. So far, my elastic ones are working.
I installed a "Speed-Feed" buttstock on my son's Mossberg; it holds 4 rounds total in 2 spring loaded recesses just back of the grip section, on the right side. It wouldn't do to bang around the back country with, but for a house gun it should be okay.