if you go to jumbo there are some nets that they sell and they have velcro which you put on the frame directly. very easy to put and cheap. Not sure if they will hold the cat in case it pushes much... but might be an option

I have some spare screens like those described above if you want them - got them for only a few euros on the French side of the border, but the problem is that the windows for which I need them are the tilting kind that don't really work with screens ... 

Let me know if you want them, and believe me, this Europe-wide lack of screens drives me bonkers as well (no relation to any feline-falling issues, lol).

I have the same problem.  We bought the netting from Brico Carouge and cut it to fit the window and installed it with ties.  The cat simply ate his way through the netting five mins after we installed it :-).   Brico also has a kind of  mosquito netting which can fit onto a frame if that could suit your window.  We have old style shutters so need to have access to open and close them.  Any suggestions welcome (apart from getting rid of the cat!).

Something like this?

Seconding Casuistik's recommendation, I had Felisitting install screens on my three windows and enclose the balcony with a net that still allows me to use it, but keeps my cats safe. The day I could open the windows without fear of losing a cat was a great one. 

Of course, my cats did chew right through in a few places on each of the windows, but the fellow came right out and repaired them. The nets are guaranteed for three years. I am so happy with them and so are my cats. I'm sure you could easily do something for cheaper with a good handyman (I recommend Justin), but I was happy to have the guarantee. 

you could take a diferent approach altogether and hang lemongrass and lavender by the window as these are repellent to cats it would stop them getting to the ledge in the first place and make the place smell nice. If that does not work cat repellents with fox urine can be bought at pet shops and i think chemists? Not sure!