[botsapper]

I concede not being 'airtight' on the strict boat tail theories. I was intrigued by complex F1 rear airfoil designs. They are entirely 'separate' structures highly sculpted & tuned for down force but they are also used for turbulence control. The deep vertical end plates close a 'loop' around the rear, supposedly to shape & minimize turbulences and vortices. It reminded me of closed box wing designs. Closed wing surfaces has the benefit of minimal wingtip vortices which occur with conventional wings or corner body shapes, associated with induced drag. Closed flow surfaces, eliminates wingtip or corner drag. I also envision a tighter flow @ higher speeds. A hollow boat tail, if you will. Easier to fab & still allow to access rear hatches. Rear lights visibility, iffy.