i bought a quater panel from mini spares and its the same as your photo above, i just seam welded it on the window area, and it came out fine.
and welded it along the centre body line so once it was ground down you loose it in the sight round
i know what you mean, i have seen panels with more profile around the rear window area which makes it easier to cut in
with the curve front to back, your right i does not have much of one, but once it was clamped on it seem to curve right like it should
this is the rear quater i did
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