what body style is it? coupe, sedan, hatchback?? I would say a 4x8 trailer, and build a plywood box on it to put the kayaks in, you could taper the rear of the trailer to help the airflow (if you have a hatchback you could even do a teardrop style trailer to help the aero.
http://www.travelizmo.com/archives/c...op-trailer.jpg
that gives the general idea of the shape, you could design it the be a little bit shorter in height than your car, and have a small wing to help the air across the gap inbetween