It is possible to use a Z3 front glass guide reversed as a rear guide - the benefit being that the vertical curvature is the same. Then find a top rail. Here is one welded up in my garage:
Z3 R door with frame.jpg
Also shown is a drawing of a Z3 base, in light blue, with GTB panels. They are to the same scale, but the sill is trimmed to suit and the GTB panels moved slightly, in an effort to give satisfactory proportions. ie the nose is lengthened and the rear overhang reduced. Only 3 front vents and the rear vents are moved forward a touch.
gtb panels on lt blue Z3 drawings 7 jpeg.jpg