[AndrewJ]

Heck, all you really need is an inflatable "bed" mattress and a big jar of contact-adhesive.
Get one of the inflatable beds with a built-in pump (that inflates and deflates) and cut up the mattress to make your boattail. There's plenty of material there to work with, and its rubber, so it'll glue up with contact cement wonderfully. And for tips on how to properly glue up rubber with contact cement, just watch the Mythbusters episode where they escape from Alcatraz on their homemade inflatable raft.