Although nobody I know who will do a good job, can do it for $1000.
Try asking these guys first and foremost:
Betz Speed & Color
1470 W Katella Ave
Orange, CA 92867
I don't think they do any body work themselves, but the shop has been around just about forever, and all the old time hot rodders go there. They are bound to have some suggestions of who to see.
Another shop is:
FABRICANTE Auto Body
26341 Dimension Drive, Lake Forest, CA 92630
They repair and paint Alfas that have been damaged. Not sure if they will do custom work. They do have full capability to make body panels - they carry all the old fashioned metal working tools to make parts from scratch. Extremely detail oriented. Their body work is flawless.
And then there is this company. I sold an Audi to one of their management. Most likely out of your range, since they do full on show cars from scratch. Nice to drool though:-)
If you speak Spanish or have a good friend who does, Tijuana is a good place to go. There is a section of the city that specializes in automotive work. You just have to review the quality of work and be clear on what you want, before you allow them to start work. Not for the faint hearted and you should have a very good idea of what you want before you go down. I don't know that they could fabricate a top, but they might be able to brace the chassis. Like I said though, be very, very careful. Stay away from the paint jobs. They are dreadful.
Have you considered a fiberglass hardtop like Mercedes or Miata? That might actually be easier to fabricate than a soft top. Yes, you will either have to put it on when you leave the house, or risk leaving it off, but it would make the car stiffer, quieter, and warmer.
You might also enquire around with people in the know, about who made the top for the GTC. Perhaps you could get your hands on the frame for that, and adapt it to your car.
Just make sure to get the chassis reinforced first and foremost. If you should ever be in an accident, I would rather we mourn the passing of the car, not you ;-)