The Mountain High bottle (AL682) comes with a nice carry pouch and tie down straps. I cut the straps such that I had two crisscross straps (using the four strap that came with it) that hold down the bottle in the pouch on top of the tunnel between the front and rear seats. The four strap ends go under existing tunnel screws.
Simple, easy to reach behind and turn on from the pilot's seat, easy to get in and out for refill and provides an arm rest for the rear passengers. I used just half of this bottle for an extended trip out west over the mountains.
Side note - five of us at the airpark went in on a O2 refill set up. Five bottles in a daisy chain so the lowest bottle is used first. The last four refills cost me a total of $40 (the price to exchange an empty for a full bottle at the local supplier).