Expanding foam works. Also, instead of rocks, go get some wheel weights and put in the stock. You can get the type of wheel weights that can stick to the inside of the stock for cheap money.
The sticky wheel weights work well adding weight to the forearm also on hard kickers.
Bondo in the butt stock works well, also.
I have used casting resin and fiberglass resin on stocks also, but only pour about 2" at a time due to heat.