This bike mount holds three bikes and works great on stock-size spare tires. It is a little too snug on 12.5" MTRs, though.
This comes complete with an Aluminum Wheel Scratch Undoing Mechanism (AWSUM) [size=0.7em]Patent-pending[/size] (that's what looks suspiciously like a towel secured with psychedelic duct-tape). I had seen in reviews that these could scratch the spare, so I came up with this high-tech answer - never had an issue.
$40
If those wheels and tires look sharp, you can still get them, here!