So, yet another $2 solution courtesy of Walmart. It warms my heart to find such thrifty solutions for my Rebreather!
This item is marketed as a "Bottle Bag" at Walmart, and sports a nifty quick clip and drawstring. It works a bit better for the secondary due to the wrist strap on the primary, but is still functional!
I actually sent my wife for a "Can cozy" for my Salvo light, and she returned with a pair of these. The potential application was immediately apparent!