Just an idea for a similar project on other vehicles:
Some SUVs have a "swing-away" rack for mounting the spare tire on the back of the truck.
Wouldn't that be almost the perfect mount for a custom boat-tail?
It's big enough that you should be able to easily mount to it with three points of contact, and the whole thing could swing away, so you would still have access to the back of the vehicle.
I'm still waiting for someone to make the inflatable boat-tail!
