Are you are going to use the window covers while the windows are open, for shade?
If so I have see covers that are 2 layers of fabric sewn inot a pouch the same size as the windows. The opening is at the top with 2 snaps in an inch or so from the sides. You slip the pouch over the window and snap the fabric to itself and this holds it in place. I have seen the same thing done with strips of cloth on the back but these tend to flap in the breeze.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato