Eventually when the time arrives in the build a lexan glass will be shaped up (it's a fairly flat glass for the most part) a custom glass window will be made (for money reasons, this may be unlikely) or possibly a donor glass with the correct shape could be used and cut to fit.
Keep in mind while some of the rear windows from fifties cars were laminated glass like a windshield and can be cut many are tempered glass and impossible to cut. ~ John Buchtenkirch
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John
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