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While not dealing directly with window channels, here's a link to a thread about a year ago dealing with fabric roofs.
https://www.fordbarn.com/forum/showthread.php?t=53594
Go to post #7 and look at the first picture. They're bending the trim strip that surrounds the roof insert here, but you can get an idea of what kind of setup you need to bend a crazy shape without having it go nuts on you.