Stock Karmann Ghia has a 2.75" inset with 2.5" hole and a plastic disk that fits it. The hole has that small stepped inner lip to support the outer perimeter of the disk.
Same with my 356, but different outer contour. The lower body on the 356A is flat to the ground, so you won't have to worry, probably even re-use the plug that you cut out of the hole with a smidge of body work.
The positioning for the hole needs to be in-line with the end of the torsion cap, obviously.