Been awhile since I looked over Cboy's stuff. Probably worth another look.
Reskinning a door was one idea I had. Biggest downside is just about all the vehicles in the yards today have curved glass (necessary when using tempered glass, I presume) and the guide rails are curved to match.
But it does get you a lot of "stuff".
Been looking at compact truck and/or Jeep Cherokee doors as a potential donor for the inner section and mechanicals as their windows are about the closest to flat of anything out there right now. Would be easiest to utilize on a coupe, I'd think (haven't cut one open yet). With a ragtop/hardtop, I'm not sure how the regulators/channels would work.