Great news re engineer
I did think the frame a bit heavy too given the strength of the pressed lower wall you used but agree you don't want a weak point there
Rear quarter window - why not reconsider a flat steel panel with glass inserted as per your rear window? will be a lot cheaper and easier to fix - nothing to take apart. Could use current frame as the base to fit flat steel panel to
Remember to get glass tinted before you install - cheaper and easier that way
Ski boy

