If you don't want to make something then you can easily buy a "sock" to go over open windows and keep the rear closed.

https://www.amazon.com/Magnelex-Sock-Style-Window-Shades/dp/B08J8G4SF8/ or similar countless others that fit your vehicle. When I was exploring the OZ outback I just bought some bug screen from a hardware store and "wrapped" it around the door/window and folded it over the door perimeter to seal it shut.