Any tips on cat proofing a window?

We live on the second floor and as it gets hotter, we’re gonna need the windows open. I’ve used an old airer for now but would like something more permanent and better looking. What do you guys use? Tia

We live on the second floor and as it gets hotter, we’re gonna need the windows open. I’ve used an old airer for now but would like something more permanent and better looking. What do you guys use? Tia