[skyking]

you could split some pipe insulation in half, then affix it along that harsh edge to the side wall. It is not an ideal radiusing method, but doing it in the existing metal and wood is really time consuming.
It is one of those things about aero, you can afford to increase frontal area if the CD goes down sufficiently from your improvements.
One added benefit from taming that edge is handling in windy conditions. In a crosswind ( let's face it, we are almost always in some form of crosswind ), the sharp pressure change on the downwind edge puts a huge side load on the trailer.
This transmits through the trailer tongue to impart a turning moment to the car.
My 5th wheel combo exhibits none of that due to the pivot point being just in front of the rear axle, effectively right over it.
When I tow a conventional trailer, even a small uhaul I can feel that turning moment.