Two piece screen window clip. These fit American Craftsman window screens. I used plastic model cement to glue the parts together once they were installed in the window. Make them out of ABS so they are more weather resistant.

Print it. You may have to grind some metal off the inside edge of the window frame. The frames are cheaply made and the rectangular slot is die punched out. This leaves a lip that catches on the clip. I used a dremel tool and it took about 30 seconds to grind the aluminum away. Once I did that, they worked perfectly.