Just be careful what you put in the Whisper stock. The forward end is open into the trigger area. Things like spray foam, or even shredded paper could wind up in the trigger mechinisum.
If you want to add weight to realy soak up recoil and noise, and improve your hold like a target rifle, put a chunk of foam rubber in first to close off the front of the stock to the trigger area. Then experiment with quantities of bird shot well mixed into modelling clay, or the duct seal some buy in the home centers to use in their pellet traps. You can get inexpensive modeling clay at any art store, or art department of Wal-Mart. and work it into the stock.
Paul.