When laying out your project you can use rulers, protractors or printed drafting mats to get your lines and angles the way you want. But how do you draw a reliable strait line across a three dimensional molded part? Try this easy trick.
Find yourself one of these profile gauges. You can get them online or at many hardware or wood working stores. When pressed down across your molded sheet they will form to the surface features and give you a fairly smooth edge to trace against.