Based on
this paper, (page 103/116, or 85) I'd go with this plan. It's about an 8° slope overall which will get you within 5% of the achievable maximum aero drag gain, but you'll drop your aerodynamic lift by 60% or so. In addition, you'll have a lot more practical space under the cap to use, and depending on how you design the back of the cap, you won't have to stoop as far to reach under it. Your height above the tailgate will be about 7.5 inches.
Blue line is the Template,
Green is the 8° line,
Red....well I think we all know what that would be don't we?
Let me know if you want to do this, I can make accurate drawings that you could take to anyone with a large format printer and get life size templates made.