Here's the camping trailer I built. It really isn't aerodynamic, but it's cute and fun, and lightweight, so it's easy to pull. The top of the camper is even with the top of the S-10 if I have the cap on it.
Out camping one time, I met a girl who built this camper.
Notice how the back of hers is straight, instead of curved, and has a distinct vertical portion at the back bottom. She claimed that she gets 3 miles per gallon better WITH the trailer behind her 6-cylinder Dakota than without it. I have only ever clocked one occasion where my trailer improved my economy.
Also, here's a similar style trailer I saw behind a motorcycle when I was coming home once from a road trip.
The TOP of the trailer looks like it's not back for aero, but of course those big boxy fenders sticking out really hurt frontal area.
Anyways, these are all fun styles of trailers, and I think MODIFIED VERSION of them could be useful for aerodynamics.
PS: Both of the camping trailers were built on Harbor Freight utility trailer frames. In states where you are required to have a - what do they call it? Certificate of Origin of Manufacture? It's not a VIN, but a thing saying where the trailer was made, and that it's DOT approved. Anyways, the Harbor Freight trailers come with that. So, technically, both those campers are just "utility trailers". In my state, typical trailers do not need any sort of plates, just the usual lights and safety equipment. In a state where plates are required, that certificate can help you get them.