Agreed on topper + deflector.
Also, side skirts like this:
After that the biggest gains are likely to be found at the rear of the trailer.
To reduce drag, you need to reduce cross sectional area of the turbulent wake you're leaving behind. That means a full or partial "boat tail" or add-ons that effectively taper the thing in profile & plan view. (EG. Note the tapering of the side skirts aft of the wheels in the above image.)
I've seen a photo of an inflatable, detachable trailer "boat tail" around, but can't find it at the moment.