www.oxycheq.com sell a cell holder. Don't know if its an official one or not, but it looks pricey ($465US).
Why not fit a p-port to one of the counterlungs (£20) and get one of the Oxycheq P-port cell holders ($30US)?
Still, it seems a bit odd that the VR3 makers trust 1 cell to be reliable enough to be calculating deco when the RB makers think they are so unreliable they need 3 to run O2. Oh well...