Gahhhh---Don't stick something down like that! Keep in mind the thing we're shooting for is making the shape as gentle as possible in the back. Just because a tailgate up is a modicum better for fuel efficiency, it does not make it an efficient enhancement....it's just the lesser of two evils over having the tailgate down. By building an aerocap, you are way ahead of the game. Hanging something in the slipstream like that would not be too different from putting a drag chute on the back of your truck.
If you really wanted to go all out, take the tailgate off, build a longer cap and hang out the back with a lower "Diffuseresque" thing dropping to the bottom of your bumper and coming up from there.
I can't speak for what angle would optimize the efficiency here on bottom, I know for sure you want to stick with the top of the template curve.
If you want, and you keep you current design, you could raise your cap a few inches in back without hurting the efficiency of it, my analysis of your shape shows you could shift it back a bit which would move it up. It may not seem like much, but 2 inches will make for more room and easier access if you build in a little swing up door onto your cap.
Here's what I mean.
