What a timely post, I am doing mine now.
My frames are good so I am redoing them, two coats cpes, then three coats Awlwood primer, sanded to perfect, then Awlgrip primer and topcoat.
Frames look like molded fiberglass.
Considered making frames from extruded fiberglass. With carbide tools cuts like wood. Then glue them together and paint.
With a little thought, you could make a trim to clean up the hole, then glue the glass in. I would have done this but don't really have the time. It would make everything very clean looking, but windows would not open.
On mine, almost each hole was a different size. Looks like they custom fit each one. Cabin has a slight bend also, very slight. Might fit them and finish one side so it will hold curve before removing.
Post pictures of what you do. Also check IGBO site.