How do I make a plug symmetrical

I have been talking to a few people and they also explained a system that has a center tube that runs from front to back that sits above the body. On the tube is a arm that can be swung from one side to the other.

Off of the arm is a measurement device , this allows you to go back and forth checking distances quickly

Some auto frame straightening machine have similar devices but these are very expensive

 

You can simply use two straight edges parallel to the center of the car. Make marks on both and use a third straight edge over them. Measure vertically from it and transfer the measurements side to side. Small differences won't show to the naked eye. Another method, if the shape is made of foam, is to slice it into parallel slabs. You can them make patterns and transfer them side to side. Cut the foam to the pattern and glue the slabs back together. Usually, you start with slabs so it makes it easy to use the method.