What are know as "teardrop trailers" really aren't very aerodynamic.
I wanted to point out the crazy looking red and blue trailer pictured at the beginning of this thread because it has a different back-end than most teardrops - one that is more aerodynamic than usual.
This guy built two different teardrops - the second one with aerodynamics in mind.
I have also thrown around ideas of a generator trailer for the electric motorcycle (which could also go on the Metro, if designed right).
If I ever did create such a thing, I would definately want to build it with aerodynamics in mind.