Depends what you want, opening, fixed, seamless, etc.
I redid my GB windows last year. An incredible job to do right.
Knowing the pain I was about to endure, I explored many alternatives except aluminum.
The best choice is extruded fiberglass, cut and routed to an almost flush shape. It can be glued or screwed down. Easy to paint and seal. Cheap materials. Almost did my GB with it.
Next is wood. Mahogany is long lasting and very forgiving, easy to work.
A good cabinet maker can use either material and build exactly what you want. Frames can be proud or flush mount.
I used urethane sealant, Awlwood primer to epoxy seal wood, and the Awlgrip primer and top coat. Looks exactly like fiberglass, can not tell where wood and fiberglass meet.
It is really just a man hours thing. I am very fast and efficient, still took about twenty hours per window, start to finish. Much faster if you don't bung seal the screws.
36 Grand Banks Europa