[aerohead]

If the amount of use and distance traveled justifies the cost, a contemporary, best-case commercial product for ideas would be Peter Brock's 'Aerovault' trailer, @ bre2.net/aerovault.info.
Peter designed the Corvette Sting Ray and Shelby Daytona Coupe.
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Also, U-HAUL trailers utilize aerodynamics researched by NASA in the 1970s. Their fenders suck, but Brock's can be an example of a solution for that.
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AeroStealth did a couple of You-Tube videos on trailer aerodynamics based on the NASA data.You might search for John Gilkison Aerodynamics videos.
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Ideally, the trailer and Jetta would be the same frontal area where they meet, with no gap ( articulated buses and railcars ), and then boat-tail from there back. And then reality intrudes!
You're the only one who knows what you're willing to put up with.
Make sure the pain justifies the cure.