I tend to agree with Bob -- the trailing shape of the tailgate would be "cleaner" I think if the entire surface of the cap was tapered down; so the air flow over the truck as a whole was improved. Like the
shape that others have done -- though I do like the look of this project. So, maybe a combination?
I'll attempt something is SketchUp -- which has an excellent free version, by the way. And, this would deepen the ribs nearer the back, too I think?