You could take the "hull" (read - rear fender) offsets from a known plane - just as thuffner3 suggested, you could use a laser level (a good excuse to buy one) and follow the example at:
http://www.newavesys.com/HullReverseEngineering.htm
It walks you through a larger project of reverse engineering a hull - but you'd be on a smaller scale and have some tighter points/tollerances to get a good fit. Principle the same tho, wouldn't need to use Prosurf - just your cad program of choice.
Jim
Simpler said - set a plane of reference, measure offsets from the object to the plane, transfer the offsets to CAD, press on.