I found 'em...
http://www.tirecradle.com/
...for $300 a set.
Ow.
Apparently they're not curved plastic - they're some sort of polymer that, I assume, compacts and spreads out the pressure instead of mashing the tire flat in one spot.
Why doesn't somebody just make a set of curved plastic cradles? Yeah, you'd have to mold different sizes for different OD tires, but for $50 apiece you could undercut this guy by $250.
Do any of the engineers in the club think this wouldn't work?