Home Depot has these labeled as 'Everbilt Filp-up tool hangers' for $6/ea and they'll hold two bikes dropped inverse so the bars touch the seats.

http://www.amazon.com/dp/B0000DH4NM/

I guess you could buy on Amazon, but it's cheaper at Home Depot and that one uses a more textured, dull rubber I prefer over the shiny crap.
[MENTION=185430]dddd[/MENTION] that is seriously awesome. Are you using a double-hook, or does each bike have its own?