Made it out of some schedule 80 PVC with end caps machined and glued in ( same as my light cannisters).
A thread was cut for a P port in one end ( I use P Port cell holders in my IDA-71) and a 100KPA gauge in the other end.
A simple BC quick disconnect fitting allows for O2 input and a OPV valve stops excessive pressure build up.
The whole system allows me to plug O2 monitoring direct from my rebreather so either rEvo Dream with 2 cells or VR3 with a single cell. O2 supply also comes from rebreather system.
All in all it cost me $50+- in parts and several hours on the lathe/mill
Making a similar system that does away with the P Ports and allows for lose cells inside that could then be read of a Shearwater or VR3 would be simple enough to do as well. It would just require a threaded or latched lid and a Fischer cable.