Welcome to Ecomodder!
I posted this picture a while back and someone actually implemented it on a travel trailer. I think he used 4" stovepipe, the split kind, unrolled and attached to the front edges of the trailer. It's a way to get the radiused edge without intruding into the box.
Beyond that, race car wide-body fenders over the stainless steel ones. And see if you can channel the air from under the tow vehicle to under the trailer, or at least divert it to the sides.