The pattern is shrunk bubble wrap.

This is the body frame covered in bubble wrap which has been shrunk to fit with a heat gun. The bubble wrap is about the same width as the final covering material, so this is a very accurate pattern. This gives a quick and cheap way of giving a surface to sketch ideas on to before committing to the final and harder-to-come-by Dacron. The red/brown lines are electrical tape which have been stretched on to give the cut lines.