How about something like this?
It's a very crude drawing in paint, I'm at work so the computer has no software for anything else...
The red part is fixed to the cab/bed and not moving, covering the gap, making it possible to leave room at the front of the stack for opening the cover, while still covering the gap... I'm no aerodynamics ace, but to me it seems reasonable that the air should "see" this as the same as the cab being longer? And considering that the steps and the "bar" overlap the gap should "disapear" in terms of aerodynamics, right?
And it should still be "short" enough to give good rearview...
Just a though...
