This is the inflatable boat tail which Freebeard mentioned.
It's basically a wooden box which encloses a 12-VDC blower fan which breathes through to a tailored bean-bag envelope,stapled to the perimeter of the box and attached to the tailgate.
It did require a stabilizing spar affixed to the bottom to prevent oscillation.A simple electrical umbilical cord ran under the truck and simply spring-clamped onto the battery terminals during testing.
It's very light and with the 1/2-tonneau and the cargo box in the bed,it gave 5+ mpg at constant highway speed.
The Naughehyde envelope created a square-to-rounded corner transition which kills attached longitudinal vortices and allows for the ring-vortex.
