We have a 36FK as well and had the same problem. I bought this on Amazon for $8:
Camco 45167 SunShield Reflective Door Window Cover
It attaches with Velcro around the perimeter of the window. It has stayed in place for going on 2 years now.
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Ty & Deb Montag
2015 Jayco Seneca 36FK
2014 Honda CR-V toad