Interesting problem to solve.... How about using a thick plastic or metal car-sized plate blank (imagine something like this: "American Patriotic Allegiance Personalized Plate Frame
") that you simply drill four mounting holes into the mount the motorcycle-sized plate on? The holes in the larger blank would allow you to mount it to the trailer. You could use the silver or black background, or the grayscale flag as a background. It might work....
Or someone else might come up with a good engineering solution. The only other plate adapters that I saw have rows of grooves and holes that are really ugly looking. I'll keep an eye out for some other technical solution.
Hmmm... how about just a mounting bar that mounts in the top two screw holes for the automotive plate, but allows you to screw in nuts and bolts to mount the motorcycle-sized plate to it? Something like this flip-up bracket: "Flip-up license plate holder
" could allow you to drill a couple of holes to mount the motorcycle-sized plate in the middle of it, but hold it very securely to the car-sized plate mounting screw bosses on the trailer.