Working from my old Kindle is not making this easy. Copying pictures to post isn't very easy.
If I remember the picture that has the pickup truck designs, the cab spoiler/half tonneau was nearly as aerodynamic as a full aerocap. The D shape I'm thinking of is ironically, very similar to the shape of a roof spoiler built by Kachi22. I think he links that thread to most of his posts.
Just to throw another twist into this mental exercise, I found this quote while following different links from the Kachi22 links.
"a shallow (20 degree) spoiler as long as 60mm is capable of actually reducing the car’s drag while reducing rear lift or creating modest downforce".
It comes from this thread.....
Rear Spoiler on Pick Up Truck - Experiment - Page 2 - Pelican Parts Forums
Now, I wonder if this type of spoiler on the end of a half tonneau (or extended half tonneau) would have the benefit of reducing drag without increasing lift.
Information overload.........help !