Last year I had a local window shop replace the very crazed rear window and both front and rear weatherstrips in my '83 Scamp13. The front window was in excellent condition due to living under the rock guard most of it's life, so was reused. We measured the thickness of the FG shell and the plexi, then ordered the correct weatherstrip, same stuff is commonly used in boats and stepvans.
On the appointed day, I took the scamp back to the shop, where he immediately popped out both windows. The original rear window was used as a pattern to cut a new window from identical thickness plexi. Once the window was verified to fit in the opening, the new weatherstrip was test fit on the windows and cut to length. The last step was to glue the weatherstrip on the window with black polyurethane window adhesive, filled the outer channel of weatherstrip with poly adhesive, then snap the windows in place. He made it look easy.
They are super solid feeling now, and no
leaks, unlike the old petrified weatherstripping. Total cost was <$300 for one plexi window, 20' of weatherstrip, a tube of black sikaflex, and 1.5hrs of labor.