[aerohead]

Some thoughts about the photo:
*As low a body as you can stand.
*The roof moves away (hinged or parallelogram)for loading/unloading,then lowers and latches for road.
*The roof has 2-blisters (ala Abarth roof) to minimize frontal area,just clearing the bikes when latched.
*Bulbous nose
*Sideways canoe fairings over the fenders
*bellypan if you're so inclined
*Any plan-taper that you can get away with on the upper body,above the fender elevation.
*If the back of the Jetta has the Kamm type roofline,it would help kick the air over the trailer.
*If the Jetta is a notchback type car,you might make a faux-Kamm device to kill some of the void between car and trailer.