My 2 cents. Make an endview template that matches the curve of the excisting window you want to cover,this can be used as a curvature gauge.
Take the gauge to a sheet metal shop and have them run new sheet metal through their rollers until the curve matches the gauge. They should be able to make make 2 pieces slightly larger than the window. For the inside shell it should be the same curve. So if you had 4 of them, you'd have 4 nice patches to do the job. Hope this helps.

1951 Canadian Chev 1/2-ton Shortbox