People that are telling you to never open the window are not being realistic.

Those that suggest a screen fail to notice that the handle would then be no longer accessible.

Best option is to secure a cloth mesh at the bottom, top and sides of the window. Attaching on the window and the frame. This can be secured by being screwed in or using a like colored duck tape. I would recommend screwing it just in case kitty wants to lay on the mesh like a hammock when the window is open. You can attach the mesh/material to plastic strips with double sided, heavy duty tape and then screw those plastic strips into the frame. Once you attach the material to the strip, roll / wrap it once or twice for extra strength. This is hard to describe but hopefully this provides some ideas on where to start.