I totally agree with Nick here. Your trailer essentially drafts behind your car in NASCAR and bicycle racing. An easy way to tell what is causing an increase in drag is to look at the front of your car dead on and see what sticks out above and on the sides.
As far as designing some type of cowl to increase your fuel efficiency it is extremely difficult and I think you’d end up spending much more building the stupid thing than you’d ever see in fuel savings in a hundred years.
Awhile back there was a group of the best of the best engineers working for Porsche that had supposedly used all of this new cutting edge technology to cut down on drag on some new sports car. When they did the wind tunnel tests they found that they did cut down on the drag. Just for fun they put the car in the wind tunnel backwards and the test results were phenomenally better with the car backwards than forwards. Aerodynamics is difficult for even the best of the best.