you will not at much weight to the butt stock if an assortment of pvc tubes are inserted before pouring in the rockite. just close the inside end of the tubes with tape, and use temporary weights inside the tubes to keep 'em from floating up as the rockite is poured in. lots of stock stiffness, not much added weight.

this multi tube thing also gives the advantage of adding weight inside those tubes, if ever needed for balance or recoil control.