I was assuming the same direction of travel, maybe a little less caster.
http://ecomodder.com/forum/showthread.php/boat-tail-project-based-single-wheel-trailer-26997-25.html#post559982
Angel And The Wolf — If you're modelling at scale, there are good suggestions at AutoSpeed:
AutoSpeed - Zero Cost Modelling of Space-Frames
I've reposted this a dozen time but maybe you haven't seen it. This is a collapsible boat tail for a VW Type II:
It features a rigid floor for the underbody flow and a fabric and fiberglass rod upper. If this had a turn-table like BZPs Dark Aero and a segmented upper structure, then you'd have what you want.
I watched him struggle with a single attachment point at the bumper. You want a wide-based hinge line; and then a high-tailed pant over the single wheel that has a license plate and 3rd brake light on the tip.