I just got my first rear bag. It has some soft cowhide on the ends, the rest is cordura with dimensions about 3.5 x 2.5 x 7 inches. I dont have the grip strength to squeeze the bag and get significant movement. The bag I bought has a rubber fill that has a good weight to it, but the fill is cylindrical, not spherical. I feel like I can find a better fill that will let me manipulate the bag more.

I understand it's a trade off with weight and stability, but I need it to have more movement from my limited squeeze strength.

Does anyone have an Idea for a filler? Cheaper is better, it's an Amazon bag that I'm trying before I invest in something 6 or 7 times it's cost.

Try airsoft pellets.

Seconding Airsoft pellets. Easy to squeeze and shape, but stays pretty much in place once adjusted.

My wife bought some pretty cheap plastic pellets from Hobby Lobby and some camo fabric to make some custom bags for the range and deer hunting from a stand. The pellets are normally used for crafts like stuffing toys and dolls. They ended up at the Goldilocks weight for me, just right.

Armageddon Gear game changer.
Get the small one with the heavy fill.
Problem solved.
Waxed canvas is easy to maneuver to get the SOLID anchor you want on the rear of the stock when shooting off a bench, lays flat to shoot off of virtually any kind of rest for field work.