Go to your local automotive repair shop that does brake repair. Take a 5 gal bucket with you and ask them if they would put some of their "dust" (metal filings) from their brake lathes. You will only need a few scoops. You know, enough to fill the cavity. Mix up some 2 part epoxy and then add dust. You will want mostly dust, but enough epoxy to hold the dust together. It should be about the consistancy of morter or automotive bond dough. Pour the mix into the cavity until full and let set for a couple of days. The metal will provide the filler and the epoxy will hold the dust together and also bond it to the stock. This will also add some weight to the stock and help reduce recoil.
I use this method to bed my actions into my stocks. It gives me a very strong support for the action. Better than JB weld and much cheaper.
No such thing as a stupid question. What is stupid is not asking it.