Len has a great little rear bag here in the LRH store, another one is the 4x6 red tac bag that Sean at Defensive Edge has.
I made one to fit the longer bipods (I used the one Len sells on here as a rough pattern) and filled it with poly beads. lighter than sand and adjust with less effort.
Keep in mind the animals we shoot for food and display are not bullet proof. Contrary to popular belief, they bleed and die just like they did a hundred years ago. Being competent with a given rifle is far more important than impressive ballistics and poor shootability. High velocity misses never put a steak in the freezer.