We use a tin yoga mat from the Decathlon for the side windows and strap them down with mini bungee cords. On some of the window sills, we use little hooks on others we use the interior fasters with some line. The little hooks are from PVC and this was heated to shape it.
The tailgate is a slightly bigger mat, it is also a bit thicker and we split the mat and hot glued some corsage magnets inside. These magnets stick to the metal of the tailgate.