How much does weight matter? If light weight was more important than aero, nobody would do aero. If weight high up mattered, nobody would use a wing. Aero is heavy, but provides clear benefits.

For sure a fastback weighs more than a hardtop, but what’s 6-8 lbs when there might a large reduction in drag and an increase in wing effectiveness?

If you look at my second construction photo, you can see how narrow the fastback is at the rear edge of the side window. Compare this to a standard hardtop - look at the huge gap there. This was a key design element. Not only is there less physical material (lighter weight) but less wind goes into the cockpit from the open window. Air goes right past the window and reattaches (or so I believe).

As for materials, fiberglass on its own is heavy. If you want light weight and rigidity you use surfboard construction - lightweight core with thin skins of fiberglass or carbon on either side. I build boats as a hobby, and so I have those kinds of materials around. The only reason to use solid fiberglass is popping out multiple copies, which is a cheap and easy way to do it, but always going to be heavy.

It would be interesting to see more fastbacks. I don’t believe the factory-shaped hardtop has good aero unless you’re simply comparing it to an open top.