Great find!I'm only guessing here but I believe the plywood is too large. In the photo below there is a flange that the rear window fits into, I would image the fabric was glued to the flange and glued into the flange as well, than a reinforcing tape was applied on top the fabric over the the flange. Section G-G in the attached pdf shows the details of the full scale.
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I sprayed several year old stuff on the Super Cub last year, possibly 10 years old. Came out great.
Woke up in the middle of the night realizing I can do a tracing of the window frame and transfer the pattern to the fabric….