Here is what I did. My back window in the bedroom was an emergency exit window only, like yours no screen.
I bought plastic "pet sliding screen material" at Lowes and cut it to size of the window frame. I then used velcro and attached the one side to the four sides of the window frame and the other side of the velcro to the four sides of the cut screen.
I then added a chest hinge that swings out and props the window out. I also have a large piece of oak that I can prop against the window frame to the window on calm days.
Now I can pull back the screen, open the latches and use either the oak 12" prop. or the chest hinge prop the window open and then press the velcrowed screen to the velcro window frame.
Worked like a charm for a few buck too!
IM me if you have any questions.
Good luck
