Shadetree, I don't know whether to thank you or not.....what do you mean by the last paragraph? "If you want to read it from the experts..."
The spoiler improved mileage on that hideously shaped Pinto because it had a gigantic Vortex Generator for a rear end. (If you research this shape you'll see what I mean.) You could put a 12 inch diameter log across the area above the tail lights and it would improve the airflow off that perfectly shaped, worst possible way to design a rear end. So I dunno why you bring a spoiler mounted there to the table.
I understand that boat tailing the VW Bug would help a lot. There is no denying that. You can keep posting about it as a solution to the problem Morgan has put before us here, but you do so at the risk of violating his "relatively easy/simple modification" request I seem to remember him saying. I think he's looking to avoid having the "cut his car in two, throw away most of the rear half, then, rebuild it so its 10 feet longer and tapered" modification here, tempting as it may sound.
Why do you not feel a roof spoiler mounted at the back of his roof above the rear glass as I suggested would work brilliantly???
Truth be told, I'm seriously contemplating trying to improve the aerodynamics on my new car by doing the same thing. The only thing I see as an issue is the air coming off the c pillar wrapping into the area of the rear windshield. But I suppose in theory, if the rear air is creating a more dramatic low due to the lift, the stagnant are created by the roof spoiler would be higher pressure.
I'll close the door on long trips.