Matt, Congratulations on choosing to fly with O2.  An 11 cu ft bottle might be short enough to laterally strap mount on the fwd face of the main spar - in front of the seat.  What the heck, we are sitting under 9 gals of gas, might as sit over the oxidizer.

You want the valve within easy reach and the gauge within sight.  If you place it in the baggage compartment behind the co-pilot seat, remember to consider constraining its mass should there be a crash, turbulence or negative Gs.

Enjoy flying high,

Scott Krueger
N92EX