If it is a portable bottle, I wouldn't put it aft of the baggage compartment. I think the above post is far better. If it is permanent, then you could put it anywhere aft but I think further forward is better. I didn't have much choice because of the A/C ducting one bay back. Here is a picture of the mount I fabricated:
Mountain High's recommendation was to mount the bottle horizontally. I think the logic was that in case of a vertical deceleration, a vertically mounted bottle might slip though the mounts. By that logic, mounting laterally would make the most sense but I didn't have room for that and I think the baggage bulkhead would stop the bottle - if the forces are greater than that the problem is probably moot.
I mounted the fill (and the ground power, no they won't be used at the same time) under the floor of the baggage compartment with a hinged cover so that I wouldn't have to put an external access panel. They are very accessible through the baggage door. Here's a picture of the arrangement before I finished the floor:
These are, of course, just ideas. You will be, in the end, your own engineer and designer.
Patrick Kelley - Flagstaff, AZ
RV-6A N156PK - Flying too much to paint
RV-10 14MX(reserved) - Fuselage on gear
EAA Technical Counselor #5357