I'm in the film production business and I use sandbags on every shoot. An inexpensive and durable way to have some versatility in having a stable rest is to use sandbag made for the video & photo world. They have straps for carrying as well.

Here a link to Amazon with a number of choices.
http://tinyurl.com/gunrest

I use mine all the time in different configurations. Some times I lay a small one across the barrel to prevent barrel jump.

You can fill them with anything you like sand, lead shot or rice (which I don't recommend, since it is subject to humidity and moisture.) Non-Clumping Cat Litter also works.

If you want a lighter weight bag use Sandblasting Glass Bead Abrasive. It's about $40 for 50 lbs.

Sand and Cat litter is cheaper.

__Thom