Might have to use multiple layers glued together and sanded to the correct contour. Time consuming, but it works. Your top piece could be like 3/8" plywood. With a bit of water and lots of clamps it will conform to the template and sandwich the 20ga. in between....

As you're fitting the plywood template to the opening, remember to leave a gap between the template and the car to allow for the new metal to be hammered over the edge of the form.