The reason I asked because it looks like it was never taken off. But I know you stripped the car down to just the frame, but don't recall a pic of the window being gone. When I had my car painted and I took out the side windows, door trim and all that happy crap, i read that it is a real B to remove the window and if it ain't broke, leave it in. I think one of the reasons was the heated glass. When they did tape up the rear window, it had just a few lines of overspray on the rubber trim, but not that noticeable.
Good luck with that rear window trim creation brain storm. I know you'll share what you find that worked.