I actually did get one of those small PHODS systems - thanks for posting that link, it was a great find! - and plan to use the metering device, but not the bottle nor the regulator. I expect to use a larger bottle so that I don't have to fill it as often, and it's the location for that which I'm concerned with.
I found a couple options for a remote valve/switch device and can do that if I must, that also was a decent idea, thanks. I'd prefer to have a manual valve that's accessible.
I won't be going to Oshkosh.
So far the main choices that might work are, in the order I recollect them,
a) Under the seat cushion, where there's plenty of room but visibility or access to the side is quite limited, and it would also mean some interesting cushion-support work.
b) Just forward of the wing spar on the centerline. Here, there's clearly ample room, but this is the only cockpit-accessible general-purpose storage area.
c) Behind the seat or behind the baggage-compartment aft bulkhead, and either using a remote system or turning it on before flight and off afterwards. Here, I'd have to land to get to it if I forget, and since I don't need oxygen every flight, that's a possibility.
d) Diver's regulator - I'll have to look into this since I know nothing about it, not a whit.
e) Forward of the panel, mounted vertically or more likely tilted forward, with the valve and regulator at the bottom so they'd be marginally accessible.
Dave