Go to a welding shop or a medical supply shop and BUY outright the biggest Aluminum cylinder you can- a Super "D" or 50 cu. ft. Why? Because a fill of the smallest compared to the biggest runs the $18.00 up to $30.00. Getting the fill is the inconvenience.
Buy a stepdown regulator (welding type) for ~$40 and put it on top. Buy an 8 foot piece of 80 PSI hose and the appropriate CGA 540 fittings (or have the shop do it for you) and run from the end of the stepdown regulator to another CGA 540 fitting. At this end, buy a Skyox regulator and install it. The Skyox regulator is expensive ($200) but is a precision machined device that automatically doubles flow if two are plugged in, triples for three, etc. It doesn't leak and it's near bulletproof (piston, not diagphram based).
Now you have a tank strapped down in the baggage compartment, and complete control in between the PIC seats.
This way you have 1) huge O2 supply, = 30 man hours at 18,000 on cannulas e.g, more than you have fuel, and on some trips you will be able to make the return w/o a refill.
Fill it at your local welding shop. Yes, I know all about ABO, but they all come from the same 25,000 gallon tank. Even medical oxygen, too.
I carry two such tanks. That's O2 for six at 17,000 MSL for 6 hours, and enoguh to get back when the FBO wants $130 for a fill.....fill at the welding co. run about $$20 since I published the above. Label the take "oxygen USP". It'll come from the same tank that dispenses ABO. Seriously.
That tiny little timed-out-need hydrostatic tank likely won't pass and will cost beaucoup $$s to repalce.
