I replaced a missing screen in my house and it kind of just fell back into the house. I guess the old screens had some spring mechanism built into them to hold them in place. There was a little groove that runs around the entire inside lip of the window frame, so I designed a little retainer clip to hold the screen in place, with a notch on the front side, so it would be easy to remove later if I needed to.

Yes, I need to clean my window... leave me alone.