Here is Germany we have window nets that you can buy for around 3 Euro at the drugstore. It comes with a coil of velcro with a sticky backing that you cut to fit around your window sill. Then the netting sticks to the velcro, you stretch it on the velcro, and then cut to size. This works best with inward opening windows, because then you can attach the net to the outside of the frame and still open the windows. Maybe you have something like that in your country.