I have always used sand because it's inexpensive. If I think about it, it's always been lacking one thing, that plastic bag! I've heard of using kitty litter but my thought is it's subject to getting wet and then it's history. Also have heard of using rice. Kitty litter and rice both have one thing in common, you have to pay for it! I too make my bags from jean legs mostly. another great material is MTY shot bags. If shot wasn't so expensive it would probably make good filler.The shot bag's with sand in them are great! I think that the best bag's have some weight to them and still have to flex but not over flex. Those plastic filler seem's to me would move around to much. Kitty litter may or may also. Even shot might. Sand takes a position under the rifle and doesn't move at all!
A problem with my sand bags is that it freeze's and the bags are in the Bronco, then you'd have to shoot off a brick! I had to sew up a few hole's in my bags last week, pain in the but but easy enough done.
Gonna make me a new one one of these days and put the sand in a plastic bag, that does sound like a great idea.