Sliding Window Locks (2 Sets), Secure Window Lock with Key, Window Locks for Up and Down Windows & Horizontal Sliding Windows, Adjustable Window Lock, Sturdy and Durable, Does Not Deform.

I went through so much trouble shooting this exact same issue- my biggest fear is my cat getting outside, he’s made it his life mission, and I too have tall vertical windows with flimsy screens and frames. I personally did not feel confident enough in any of the Velcro solutions that were available a few years ago. It does look like they’ve improved since then- I just can’t speak to how well any of them work. What wound up working best was buying additional bugscreen inserts like these. Whenever the window is open I stick this in front of the crappy landlord-issued bugscreen, and adjust the window so it’s secured tight. Maybe you won’t need to take extra step- but my clever little bastard started trying to pry the window open a little further so he could knock it out. I had some of these locks on hand which put and end to that- but a security bar (or anything else that fits snug in the frame) would do the trick as well. Might take an extra two-minutes to crack open the windows, but it’s worth the peace of mind. We’ve made it a few summers with this set-up, and no issues or scares! I’ve seen him barrel full speed at it because a bird was hanging out nearby, and it didn’t move an inch. I do watch him like a hawk on the first few warm days because he’s definitely looking for ways to outsmart this system- hasn’t figured it out yet :)