Oddly enough, an RX-8 spare
Oddly enough, an RX-8 spare WILL fit in the trunk of the NC if you remove the tire repair kit from the straps which hold it in place.
I'm working on one of these as an option myself and have been looking at options, including building one. What I've found out is that the spare tire carriers like this require a 2" receiver. Having a hidden hitch from Bethania Garage, they offer a 2" receiver. Otherwise an adapter to convert from 1-1/4" to 2" receiver would be required. Then the spare tire carriers are too tall. They're all approx. 24" or slightly taller and with a tire mounted would make them 36" or taller. They require cutting down or modifying to fit this low and some come out of the receiver and turn DOWN which means even less ground clearance.
Just some information from the peanut gallery many years after this was posted.