Years ago, I bought ordinary hooks in an ironmonger's slightly shorter than the width of the cat's head and fitted these, so the window could be opened to let air in, but the cat could not get out.
Alternatively, we used a metal grid (actually an Ikea shelf) that just happened to have the same dimensions as the window.
On board our motor boat we use strong nylon fly screens, and when we discovered that the one cat had worked out how to open them (!) we added a small hook at each side of the door frame and a loop of strong sewing elastic from the fly screen round the hook. To date that has defeated both cats. They quite approve of the screens as they can see out through them, but dogs walking past in port can only smell them, not see them!
Is it me or am I getting mixed messages
Painting Warhammer miniatures dwarfs/Elf's etc. Old World