I built and mounted the balcony, between work and other stuff it took me about two weeks. The netting itself is reinforced with wires and cannot break with just her weight, claws or chewing - you'd need at least some strong scissors to cut it. The board is supported by shelf holders mounted to the wall underneath, but actually the net would suffice. I threaded the net onto the frame all around. The frame itself is mounted to the wooden window frame with 6cm screws (maybe 5-7mm thick). They are what's really holding the weight, along with the window frame. Edit: that thin, loose netting is really just lying on top of the board and is meant to make it /look feel safer for her, like a guardrail - because initially she seemed very cautious of the height. She did indeed use it more after I put it there.

For those interested, here's a more complete picture: the frame surrounds the whole window, hence the netting is closed at the top. The board has hinges and folds up, which was actually necessary to be able to add the shelf holders.