If you don't have a new-old-stock or aftermarket replacement panel to weld in you have no choice but to bend your own. It's hard enough to do this time-consuming exercise without making mistakes, and wouldn't it be great if there were a template?
Templates are extremely useful, but trying to make a paper or cardboard one for a curved surface is out of the question. Instead, why not use thin aluminum flashing? You can get rolls of it in various widths at your local home center and it's quite cheap.
Once you cut out a piece of the flashing you can easily shape it to the contours you desire using your hands and a pair of scissors. When you're satisfied that you have the shape correct, set it aside and use if for reference while you're bending and shaping the 20-gauge steel you'll be using on the body.