What I would do is a half-tonneau, their almost as efficient as a full aerocap. Except for having all the weight behind the rear axle if you use it for lockable storage it would be advantageous. The airflow is 6-8 inches above the tailgate typically. You could knock the corners off or round it.
In the front the stagnation point rules. That's the point where the air has to decide which way to go. Adding frontal area that far back will have little effect. It needs to be furthest forward. Which leads to problems with the approach angle.
Consider an air dam that only covers 90% of what hangs down. That should reclaim an inch or two. Then leave it long in front of the wheels, with a notch in the center for clearance on unpaved roads.