Well, you posted under the "Backpacking" section, so one assumes that you are either not firing on all cylinders or else want something light weight.
I gave Brain the last of my Polystyrene beads. That is what I use.
Depending, upon your ability, you can take an old camo shirt and cut the sleeve off and sew up one end. Then cut the other end of the sleeve so it has a pattern for a filler tube and sew that up. Fill up the "bag" with beads. Take some construction braided twine and tie off the filler tube and leave enough to tie a loop in the twine. Use a snap link to attach the bag loop to your belt or pack.
How much you fill the bag depends upon your preferences in a rear bag. I carry two so I like mine loose and floppy. At ranges past 1K I do not like to be squeezing the bag because it introduces movement in the rifle that cannot be tolerated at those distances.
Now then if you have a loose sparkplug wire and just happened to be in the Backpacking section because you got lost, then I will give you the following advice. I am not an olympic champion rifle shot, nonetheless, some days I do OK. I have tested the lightweight poly bags against the "Heavy sand" bags and the Heavy sand bags are the most accurate. You won't find any poly bags at 1K benchrest matches.
The Smokin Fur Rifle Club