Perhaps the size of the un-tonneaued portion depends on the height and shape of the cab. I suspect the cab wing and half-tonneau work against each other.
The picture you describe may be aerohead's waterbed inflatable.
ecomodder.com/forum/showthread.php/inflatable-boattail-896.html#post9948
I found this pizza delivery truck with a half-tonneau/spoiler.
They said it improved the gas mileage but they didn't know how much. It reaches aerohead's Template line. He added length to get a similar result.
What would you think of a lockable storage box with end plate fins?