maybe the end of a long thin balloon animal balloon could be stuffed with a bunch of shot and then tied off.  it might explode in the air.  but if you are intenting to use scatter shot to hunt birds, just do it.  techstuff hunted birds for years using a sling and scatter shot (i think he used a special rig he put together, but almost any solid pouch will do) you may even want to try using the sweatshirt trim that i mentioned earlier, becuase it will hold a little lump of shot tight, until you release it and it flies out.  or you could use any kind of cupped pocket.  you might want to try this one: (try looking at old backpacks for webbing if you don't have any)

cupped sling

click the glowing thing to go there