Hi, this is a really good study on pick up truck aerodynamics.
http://csus-dspace.calstate.edu/xmlu...pdf?sequence=1
They have different modifications modeled in CFD to test how they affect drag and lift. Info on aero-caps starts on page 68
In summary they found this out about the drag.
Baseline 0.3453
Traditional canopy 0.3157
5degrees 0.2957
10degrees 0.2894
12degrees 0.2892
15degrees 0.2987
18.7degrees 0.3091
Now remember these aren't nice compound curves, but just a straight cut.
Now as you can see 12degrees yields the largest drag reduction, however it is barely any more than at 10 degrees, and a large amount of benefit can be seen at 5degrees. After 12 degrees however, the drag starts to once again increase but not to the point of the original configuration.
They also tested a 3D curved aero-cap which gave a drag coefficient of 0.2768. This is definitely the lowest drag option with a drag reduction of almost 20%. It also reduced the lift coefficient by over 40%.
My recommendation based on this info is to not exceed "the template" at any point, and if needed just taper less. Even at 5degrees you get a large portion of the drag reduction at 12degrees, as well as an even larger lift reduction. Try to get as close to "template" as you can for the largest drag reduction, and then do the same thing on the sides if it is possible.
Good Luck! Please keep us updated on your decisions/progress.