Here's what I'd do to the design. Everyone gets all geeked up over 20° rear angle, but I think that is too aggressive. Based on a paper I read by a Cal State Masters Thesis candidate exploring pickup truck aerodynamics, you get 97% of your aero gain by going only 10°, but you maximize lift reduction. The usable space is way better with the design I illustrate here too. So in a sense, you get the best of everything, huge drag reduction, big lift reduction, and a usable design. Win Win Winny Win.
Reason you don't want to get too aggressive on the top taper, is that you start to create a large area of low pressure that wants to get filled in from the sides, this sets you up for a vortex generation situation which produces a lot of drag.
The paper I reference is here.
Paper The table on pg. 85 sums it up.
Why this paper isn't the gold standard for truck aerodynamics is beyond me. The author Feysal Adem clearly put a huge amount of VERY knowledgeable effort into its creation, yet somehow everyone bases their opinions on what looks right to them instead of the real science Feysal did.
I can only take credit for putting 2 & 2 together here on this concept. All due credit rests with Feysal.
