Hey, Jeff, if you're looking to add some weight, try Duct Seal. If you leave it wrapped in the plastic it comes in, it's eays to remove, and can be molded to fit the stock. Stuffed full, my CFX will hold about 1 1/2 pounds. You CAN use the expanding foam, just put a plastic produce bag in the cavity first, fill about 1/3 to 1/2 with foam, let cure, then carve the excess. Or do both, endless variety! Later.
Dave