[aerohead]

If you'll go over to the 'full-boat-tail-trailer thread,there are some pictorial drag tables I put together from wind tunnel test reports on boat tails.You should be able to get a sense of what would do what.
When I did the initial inflated tail on the truck it developed an uncontrollable vertical 'wag' oscillation.
I ended up placing a rigid bonded spar underneath it which killed the displacement.
Your fold down structure would provide for the support you'd need for a stable inflated tail.
When Carl Breer's team put a 'bad' boat tail on the Chrysler DeSoto Airstream test mule they were still able to pull off Cd 0.244.The Beetle is very similar to the 'beaver-tail' Airflow and I think you'd be laughing all the way to the bank with the tax-free pay raise the tail would give you.
The challenge is providing cooling air,but NACA submerged inlets could work for that.And the tail itself would allow the boxer engine to run cooler.
I got my VW bus as high as 35 mpg with the 1,500 cc,57-bhp engine.
You could use taller gearing for top gear.I still have to address that one.