Here's some ideas:
1) With my rear seat removed (and long gone!), I laid mine flat (31.5"),
then used 2 ratchet straps in an "X" configuration woven in and out of the spokes and secured to each of the factory metal tie downs on the floor. IIRC my straps were rated at 7,500 lbs.
2) Rowhard did a spare tire mount in the rear, with the tire in an upright position.
3) I believe JollyRogerVX made a spare tie down like the ones you see in the bed of pickups...tire is up at a 45 degree angle.
Check out their galleries for pics and ideas.