Hey Brian (whoops, I almost added a 'y' by accident ;~), one thing you might want to consider is double-bagging that sand. I'd hate to have a busted bag pouring sand through that hole at the front of the cavity into the trigger area of my CFX. But it doesn't sound like the extra weight should hurt your accuracy any and could potentially help it.
I'm sure it's been thought of before, but I've wondered if the same thing could also be accomplished by fixing into the stock cavity one or more of those recoil dampers that archers attach to their bows to absorb the shock of loosing an arrow. In times past they were made up of a sealed capsule enclosing a big blob of mercury that "exploded", as I recall one of the advertisements saying way back in the '70s, within the capsule, thereby absorbing some of the shock, and were mounted to the bow with a short threaded stud. A similar type of stabilizer/recoil damper is still available from several manufacturers, but the ones I have seen in the catalogs use a gel-type shock absorbing medium, probably due to environmental concerns.
On the other hand, sand is definitely more available, and easier on the wallet if it doesn't suit you.
Bryan