i went through this search insanity last june, for a deflector that actually forced air down the rear window.
I too found all the ones above and none except the yakima thingy would even come close to directing air down the rear window
we need a day at the wind tunnel.
in the past on a f350 dually with the cab height leer cap, i manufactured a small deflector from alum and it was just high enough to grab some air and worked pretty good at keeping the major dust and water off at speeds.
been tossing a few things around to make another small deflector to do the same.
we have a huge wind burble area on the rear window and liftgate, i imagine its not enough to change mpg, or ford would have made one, but it may be enough to keep rear window cleaner.
the burble does help aerodynamically speaking as it forms a bubble that allow the rest of the airflow down and around the flat side of the rear end,
maybe time for some cardboard mock ups