Chevy called theirs a tailgate protector and support bar. Not sure why Ford chooses to call theirs an aerodynamic aid. To me...it's just some styling eye candy that they decided to give it a working purpose.
If you go with the notion that it's there to protect the top of the tailgate...then putting it on the tonneau cover sort of defeats that approach. If you put it on the tonneau cover then you'll need to find out what angle will best provide you the results yer looking fer. It will take a little time/effort as well as an adjustable tail/wing to determine the proper angle.
