Sheetmetal work - compound curve

What I did in the past for such complicated pieces was to make a template out of thin cardboard, such as was used by clothes cleaners for wrapping ironed shirts around. I used it to mark steel for cutting. Just rough cut the cardboard to fit beneath the hole and trace the hole with a pencil. Once you cut out the steel, you can hand-bend it to get it in the ballpark before welding it in.