I do like the look of the Nissan cap, and it may be fine aerodynamically -- only tuft testing will tell.
I made a quick attempt in SketchUp (after downloading a free quad cab model from the Google 3D Warehouse -- there is a Dakota there, but not for download), and it lacks the arch all the way back to the tailgate (a little more trial and error would get it there):
In my estimation, the departure angle of the whole trailing edge would be better with a downward slope.