Lay this shape over the side of your blueprint. Scale it so the ground on the drawing is at the ground position for your trailer, and the top of the car in the drawing is at the top of your trailer (4'?).
Choose how long you want to make your trailer and cut off the rear end of the drawing at that point. The longer you make it, the better the aero. If possible apply the second drawing to the plan view so the thing tapers in both directions. If you're
really adventurous, generous radii on the edges will also help.
*Make sure you cover the bottom of the trailer so it is smooth. If you can, build the body of the trailer out to cover the wheels.
