How about a rectangle of spandex, wrapped around the gap between the truck and the trailer. you could probably find some 6ft wide, so one sewn seam (very easy for someone who knows how) and you could have 12 ft. that should do it. Then get some strong magnets and rubber cement them to the fabric at the front and rear edges. You could get fabric that matched your truck, or black to look like a city flexy bus, or pink petunias if you wanted. You'd need a topper for this to work though, and maybe one of those deflectors on top so the can be edge on to the wind and not get loaded as bad. If it pops off when turning, just make it longer, so the stretch can be distributed over move fabric.
