The Rapid Craft River Racer model without the cell didn't have a rear cowl at all. It may look even nicer with a flat rear cowl especially if your running a 4 to 6 inch set back. A flat rear cowl with those sides filled in to the transom will put the focal point on the engine cowl. If you paint the motor cowl and the boat similar it will all blend together.
http://www.screamandfly.com/showthread. ... iver-racerDave Elsey's new Allison has a similar look with the rear cowl on. I would not be surprised if Dave used a low flat rear cowl on his.