I used Duct Seal in my CFX, look in the electrical section at your local Lowes or Home Cheapo. If you really pack it in, you can get almost 2 pounds in there, but I think that's a little too much. I found that 1 1/2 lbs. gave the best balance and helped absorb some of the recoil. If you stick a plastic produce bag in first, it's easy to remove later.
Dave