Check these out at JCWhitney ---- might work with minor mods and the
price is right at $37.95 USD

http://www.jcwhitney.com/product.jhtml?CATID=14940

BUMPER-MOUNT BIKE CARRIERS
Choose carrier for vans and RVs or carrier for most vehicles
2-pc. design requires no drilling
Bottom supports clamp to bumper. Top section slips
into the support posts and removes easily when not
needed. Plastic-coated hangers hold bikes without
marring or scratching. Tubular steel with black
powder-coat finish. Includes 2 tie-down straps and
hardware. Instructions included.

Cheers,
Doug Alcock
'84 Westy
Toronto, Canada