Something like this that you mount upright:
http://www.jcwhitney.com/OMIX-2-AND-...1517_10101.jcw
Or better yet, like this:
http://www.northerntool.com/webapp/w...6893_200316893
So basically, you would need to somehow fix a square tube across the cargo area at the right height, and then you could mount the tire.
But, you are talking about limited space back there. Its going to be tough. I would just get a roof rack!!
Bart