Fox, in my opinion, the gap is your enemy. I don’t believe that you will lower the drag of the SUV if you do not have attached flow all the way to the back of your trailer so that the entire vehicle follows the template. Bam Zip Pow did gap fillers with his trailer and Aerohead did some experiments with aero trailers. There was a “cargo tail” on a receiver hitch on this site that was built to follow the template but did not improve fuel economy because there was a gap between the trunk and the cargo box. If you read up on Aerohead’s trailer and Bam’s one wheel trailer they were very careful to build fill panels between the tow vehicle and the trailer. These are difficult to do because of the movement between the trailer and the back of the tow vehicle. Even with the fill panels the mpg gains were only a few percent as I recall. IMHO I believe that the best you can do is build something that suits your needs and is smaller than the wake of your tow vehicle so you can minimize mpg losses.
Bam’s thread is long but probably worth reading before you spend a lot of time and money designing and building a trailer for real world use.
https://ecomodder.com/forum/showthre...ler-26997.html