[Christ]

Some people have tried saw-tooth edging on square edges, I haven't tried it myself, though, and haven't seen any evidence that it works.

Does the door open like a barn door, or does it fold down completely? (Assuming there is a door on the back of the trailer.)

Either way, you can add like a 2" extension that slopes 12* from the top, 12* from the sides, and probably not more than 3* from the bottom. You can make a sloping box out of it, that will decrease your cross sectional area substantially and help to train airflow to a close, rather than leaving a more turbulent wake.