[Bicycle Bob]

If that were my rig, I'd wrap it with Coroplast or other sheet material. It would make a bulbous extension over the tongue, as the experiment does, and flow smoothly into the sides. It would be tapered from the wheels to the roof, rather than stepped at the fender tops, to be easy to build, and to provide some extra width that would be carried to near the tail, and then curved in, winding up angled 15 degrees off the center line to minimize wake turbulence. The compound curve at the forward roof line can be done many ways, depending on what you find handy. In the truck wake, it does not have to be very smooth for the air, but the eye would prefer it.