There is a product called Trixie Cat Netting (edit: it's actually called Trixie Protective Net) which you can tack up outside your windows. I can only find it on Amazon UK but on Amazon US there is deer netting which looks like exactly the same thing: https://www.amazon.com/Extra-Strength-Animal-Fence-Netting/dp/B00HQOO7LG/ref=sr_1_3?ie=UTF8&qid=1501494924&sr=8-3&keywords=cat+netting

You just cut it to size and nail it up, so it's much easier than a gate or screen. If you can't use nails you might be able to secure it with zip ties. So long as the squares are not big enough for your cat to squeeze through!

Here is a link to the Trixie stuff I used on my windows: https://www.amazon.co.uk/d/Cat-Flaps-Outdoor-Nets/Trixie-44323-Protective-Net-Transparent/B0009587Y2/ref=sr_1_fkmr1_1?ie=UTF8&qid=1501495272&sr=8-1-fkmr1&keywords=trixie+cat+netting