Take a look at this post
https://allmetalshaping.com/showthread.php?t=684
I made the shape out of home insulation (styrofoam ) then used masking tape to get a more blended flow over a rough surface.
the flanges are usually the last process after all the shape has been put into the part, and it fits perfectly to the back of the car on all four sides .
you may want to do it in two pieces like i did they are easy to handle and you only need to have a flex pattern for 1/2 of the panel .... the pattern can be flipped inside out for the other half.
as for the gap to the trunk lid ....... that line needs to be transferred from the lid while both panels are aligned and in place on the car.
both panels are convex in the horizontal and the vertical, so the gap that looks like a fairly straight line is really an arc.
good luck Steve