Fenestrelle Expandable Window Screen – 10″ H × 19″–36″ W Horizontal, 2‑Way Adjustable Window Screen with Flame Resistant Fiberglass Mesh, Pack of 2

Well, here in the US we had these expanding screens that set into a window and is held in place by lowering the window sash onto it. https://www.amazon.com/Fenestrelle-Adjustable-Horizontal-Convert-Vertical/dp/B075X2TTPK?th=1 This is an example of one. They are quite useful, and are slim enough we would simply slide them behind my da's chair for storage. Not a permanent install, reasonable cost here in the us, convenient. I suppose one could make one by carefully measuring the space it would need to fill, cuting wood to make the frame and attaching the screening to the frame and setting it into teh window when used.

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 :)