Here's a 'Template' comparison of your Insight.The hashed area depicts the region which is compromised due to the lack of the boat-tail.
Your trailer would just fill in as much of the missing volume as you like.
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Here's a link to BamZipPow's streamlined 1-wheel trailer that he's currently developing.Bam's T-100 has demonstrated higher mpg while towing the trailer than without it,illustrating Wolf Heinrich Hucho's recommendation for elongation for lower drag.
http://ecomodder.com/forum/showthrea...ler-26997.html