TR3aguy,
Thank you for this link. I read it all the way through. Some great info there and I wonder how hard it was in the end. Too bad the thread ended before John was finished.
I saved the fuzzy channel from my frames as it was in pretty good shape. It was aluminum with the fuzzy stuff glued in. Those sliding windows appear to be nothing more than 1/8" Acrylic, or Plexiglass, the trade name. Ninety dollars for the four of them seems a little on the rich side. I think you could buy a piece of 2' x 4' x 1/8" Acrylic at your local hardware store for a few bucks and just cut them out using the old ones as a pattern. To get them out I just pushed on them at their centers until they bowed enough to just pop out in one piece without breaking. I think I can do the same thing to get them back in. Not sure I can figure out the sewing sequence though and how to make the vinyl tight.