There were small screen locks that came with our windows over 30 years ago but most have been broken or lost so I made these simple press-in tabs to fit the grooves in the surrounding frame of our windows. 

I printed using in PETG (no support) so they would have some good flex. I bumped up the perimeters so it prints pretty solid.

There's a small slot you can use to extract with a standard screwdriver in case it gets too stuck to pull out with your fingers.