Convertible Tops

I think it's not using the existing window, it's making a top with a big window. Bigger windows mean less fabric, easier to tear.

Places that do replacement tops use tops manufactured in factories using computerized industrial equipment. The install what's available on the market from a very limited supplier base.

Actually creating a custom top is an labor intensive and expensive process, generally used only for show cars. To get what you want you'll have to find one of these custom fabrication shops and actually turn the car over to them while they do it, and unless you live in California you may have a hard time finding one. Oh, and you can leave the small bills at home....you'll only need the big ones.

Here's a typical website for one:
http://van...etops.html

I noticed larger rear window was replaced by a smaller one, but it appears the small size was probably original design to begin with... Thanks for the link; what a beautiful car.