They look like they might be brilliant for a pattern or for recycling parts, but they're made with plexi in a track to open and close with the top up -- if it's raining, I guess visibility really would matter, but ... :
http://cgi.ebay.com/ebaymotors/Porsche-356-Speedster-Window-Frames_W0QQcmdZViewItemQQcategoryZ6763QQihZ013QQitemZ230284307764QQrdZ1QQsspagenameZWDVW
Or one to R&D from:
http://cgi.ebay.com/ebaymotors/Porsche-356-Speedster-Side-Curtain_W0QQcmdZViewItemQQcategoryZ6763QQihZ013QQitemZ230284310173QQrdZ1QQsspagenameZWDVW