The air deflector I purchased is rounded on the corners. I think you need to do that to minimize aero noise on whatever you build. A longer/taller one would be more effective as well. I can tell from the bug splat pattern that mine could be bigger. I'd say mine is about 18" tall if you stand it up straight. You could be very effective if you made one 36" tall and had it in a gentle arc (or at least a wide "V") about the vertical center. Keep in mind that you are trying to guide the air around the trailer and the Excursion roof-top is only about 60" wide--compared with 96" for most trailers. It is a 3 dimensional problem and you can only do so much without joining the trailer directly to the back of the truck with some sort of faring.

I estimate that there may be another 1 mpg (from aero effects) to wring out of my lash-up when towing at 70 mph. Before I try a bigger wing, I am going to look at the front bumper air dam on the Excursion. I think the bigger front air dams from 2008-2010 F-250s may fit on my front bumper. They extend about 6" below the bumper (instead of the 3" on mine). Blocking some of the air going underneath the Excursion would help as well. Pretty ugly under there from an aerodynamic drag perspective.