For a guy to do this at home, it can be a little tough.
What I do is mount the stock in a fixture that I made that hangs off the back of a mill table. I then machine the desired amount off with a .75" endmill working outside to inside in a clockwise fashion. This ensures the cutter is shearing into the material; mitigating any chips/tears.
If the stock is hollow, I'll mix up a batch of resin and then add some micro balloons made by West Systems. The resin will heat up the balloons and it'll "erupt" like foam. When the resin cures it'll make the material like a hard sponge that is very strong and very light. Then machine again to bring flush and install your recoil pad.