sailplane/ glider trailers
 

These trailers are designed specifically for these types of aircraft. They are unnecessarily long for a 'trailer', the area for the tailfin unnecessary, and better if wheels integrated within the trailer body, however as a 'first-principle' for a low drag trailer, are way out ahead of nothing.
Matched to the height of the tow-vehicle better.
No gap, even better.
Plan-taper.
Smooth belly.
Diffuser.
Wheel skirts.
An example
https://3dwarehouse.sketchup.com/mod...a429b1/Trailer

We see these all the time since the Harris Hill field is nearby along with the National Soaring Museum. They are specifically designed for carrying high performance sailplanes. A travel trailer behind a van or a fifth wheel set up would be a different configuration. There are practical limits to the gap due to turning radius needed. A flexible cover for the gap?

I've seen trailers for carrying irrigation pipe that are inverted-U frames with a gantry trolley to load and unload, and Bonnevile race car trailers that are built the same. Skin one and you have much the same.

I have a design for a cruck-framed shepherd's trailer with a retractable pole in the crotch that extends out for a ridgepole over the rear entrance steps.

Why do the wheels (flares/fairings) have to be so unaerodynamic?

This one is no better.

https://www.cobratrailer.com/?page_id=1180&lang=en
https://www.cobratrailer.com/wp-cont...ger_master.jpg

Because you off road some on a "Fetch" using not the best roads. The scab on fenders repair really easy and meet most requirements for spray reduction. The rain spray patterns suggest these trailers aren't aerodynamic much at all. That's odd since glassholes are really efficiency oriented towards drag reduction on they planes.

wheels
 

It's always been a big mystery to me!
Pete Brock has attempted to mitigate this drag on his trailer.
Perhaps they're used so infrequently, and for such low annual mileage that manufacturers couldn't reconcile the design change.

+1

And it all comes down to economics. How much are you willing to spend in initial cost of the trailer in order to save a few bucks on gasoline?

It's more like how much to rent a hangar at the airport (or even build one if you can) versus mileage costs towing. And if you want to fly somewhere else you still need the trailer.

I was checking out a similar fender wheel cover on a box trailer going down the highway yesterday. When I passed him on the other side I noticed the other side had them ripped off perhaps from some parking accident.

Trailers take a beating, I cut a slit in a garden hose and pushed it on to the exposed edge "C" rail of my snowmobile trailer. Had to push it back on several times after rubbing on stuff.

These trailers usually load and unload on grass. Pilots often travel to take part in competitions. Wheel skirts have to come off to change a tire same as a car. What gets measured gets improved. Blended fenders and a corner radius 10% of width is basic.