I've started on version 2 - the permanent version of my earlier, cruder, Kardboard Kammback:
See: Kardboard Kammback: testing a partial boat tail prototype
Following in ebacherville's footsteps, and encouraged by a conversation with Phil, I'm going to try foam core construction, based on a cardboard mockup/base.
The mockup will be tuft tested first, to make sure it works. The main difference from version 1.0 will be generously radiused "shoulders" in the transition from the roof to the side of the car - to ensure more gradual pressure recovery and discourage vortex formation.
Here's where I'm at so far:
(forming compound curves is harder in cardboard than it will be in foam!)