Here's 2 alternatives for you...not sure where you're going with the Zombie Death Machine......but I laid this out in case you wanted to add an aero cap, which would make your truck suitable for a top rear of cab to top of tailgate aerocap for optimized shape. If you want to put something on the back of the existing topper to boattail it, than the rearward placed template there on the bottom pic would be worth considering.
Bigger Funner
HiRes Version here
Another Big Fun
HiRes Version