I kinda remember going over this before when you did the roof rack test. The trailer would win out. You would have the benefit of the car blocking some air. You could even design the trailer as a boat tail extension. To bad you don't have a sun roof, if the wind was right you could Codfish to the lake
Actually, we expect nothing less than a fully enclosed tapered trailer with flexible skirts joining it smoothly to the back of the car. That, and the ability to raise the sail when the winds are favorable.
I would have to agree that the trailer would be less of a hit to FE. you could even try to find the lightest trailer that would fit your needs, aerodynamically mod it for better air flow and maybe even add some LRR tires and pack the bearings in synthetic oil instead of grease. Be sure to use LED trailer lights as well, it'll help save on battery life if you plan on trying to drive around with the alt. belt off.
hmm...I think I smell another scientific A-B-A experiment coming this way from MetroMPG!
I'm all for the trailer idea, I'm pretty sure that you can get some nice 60psi moped tires that are speed rated at over 80mph, for how long you want to make it I would go with two wheels for stability, I talked to a guy who has a trailer for pulling behind a motorcycle that he pulls behind his TDI gulf, and I think he saw about a 4mpg decreas, but could dubble his cargo carrying capasity.