Finally I feel secure to open the window!
My windows open outwards and in the bottom part. I couldn't find a way to secure them as most of the nets or fences suit windows that open inwards. I used a strong velcro (could not attach them permanently as I rent the apt. and the landlord would not accept that) and attached the fences to the window itself, and not to the frame. This works perfectly and finally I feel that my cat is safe! Will buy more for other windows before summer.