I was thinking of doing that, though my ideas are probably more complicated than they need to be.
Basically, I had two ideas: The first idea was of a detachable section on the cap itself, that would interlock with another section that would attach to my hitch receiver, somewhat similar to what
bondo has for his aerocap.
Another idea would look similar to what a few trucking companies have already done for decreasing drag on their trailers.
mcrews took some pictures in his
Aero Semi-truck butt...... thread. I saw another one of these on the way to a beekeeping supply store this past Saturday.