Hal-tonneau and half-cover are halfway there.
ecomodder.com/blog/aerocaps-for-pick-up-trucks/
A full cap on a full length bed that meets the top of the tailgate is too fast.
The example at Permalink #2 has the right shape, but nobody uses it. It immobilizes the tailgate and would work best on a slammed minitrick.
edit:
Simulpost at 2:44
I think this could have bevels at the left and right ends. The manager knew it helped the mileage, but he didn't know how much.