Some of the GRC participants who have done it before get a pool noodle and cut off a segment wide enough to line the bottom of the bag. Gives them some spring so the bricks don't jerk you down so much during certain exercises.
Right now I'm using a foam yoga block (not pictured) under them like one guy recommended after he tried it in the GRC. I'm still very much in the "seeing what works best" phase though.
Threaded a carabiner through the molle on one of the chest straps for my hydration hose today and that worked flippin' excellent to keep it under control.