It seems every time I used one of those spot weld "hole saw" cutters, I ended up spending more time repairing what I had cut through too far, so I have been using this method since.
To start the layout on the new metal, I wanted to find the center of the radius along the window opening. By holding a sanding disc on the flats and scuffing, it showed the edges of the radius...
http://i5.photobucket.com/albums/y16...Picture396.jpg
http://i5.photobucket.com/albums/y16...Picture397.jpg
http://i5.photobucket.com/albums/y16...Picture398.jpg
Then laid out a pattern to be able to measure from the bottom crease...
http://i5.photobucket.com/albums/y16...Picture400.jpg
....and transposed it to the new sheet...
http://i5.photobucket.com/albums/y16...Picture401.jpg
http://i5.photobucket.com/albums/y16...Picture403.jpg
http://i5.photobucket.com/albums/y16...Picture404.jpg
The top edge was trimmed to size and the tipping wheel used to fold over the welding flange...
http://i5.photobucket.com/albums/y16...Picture405.jpg
http://i5.photobucket.com/albums/y16...Picture407.jpg
Comparison....
http://i5.photobucket.com/albums/y16...Picture409.jpg
The bottom was marked with the tipping wheel and folded in the apron brake. Then trimmed to size, again using the tipping wheel to fold over the welding flange...
http://i5.photobucket.com/albums/y16...Picture410.jpg
It's real close right now, just needs a bit of tweaking.
http://i5.photobucket.com/albums/y16...Picture412.jpg
http://i5.photobucket.com/albums/y16...Picture413.jpg
http://i5.photobucket.com/albums/y16...Picture414.jpg
http://i5.photobucket.com/albums/y16...Picture415.jpg
http://i5.photobucket.com/albums/y16...Picture416.jpg
http://i5.photobucket.com/albums/y16...Picture417.jpg
This was made using 18 ga, I think if I had the factory 19 ga metal it would have formed much easier..