Prevent cat from falling from second story, indoors

As you guessed already, it is going to be difficult to have a solution which is both useful and good-looking.

My advice is to train the cat to return by itself when it is "stuck" there. A wild guess might be that the cat is faking the danger to get your attention.

An extra reason to go the training path is the following. Let's assume that you actually find a solution to upgrade the fence so the cat does not go through any more. But what if the cat will try to go on top of the fence, in order to look down? The risk is even greater, I assume.

Cats are experts when it comes to falling. Their bodies are built for it - their hooked claws are great for going up a tree, but not for coming back down. Their bodies and prepped and ready for impact. HERE is a great article about cats falling, and their likelihood of getting injured. It looks like the balcony may be 10 or 12 feet high? 8 feet is, for a healthy adult cat, totally fine. It gets a little more complicated at that height, but there's a chance your cat will be fine if he does fall -- and it might teach him the lesson he needs. Let's try to avoid that scenario, though.

Training is the best option here. A fence might make the cat more insistent on trying to get through. They love to hang off of things for a "view" (my cat hangs over his perch all the time). Instead, try these things:

• Put a perch or a cat bed in that area, just far enough away for him to still see over the edge, but not risk sliding through the bars. This will give him a safe place with the view he wants. A lot of cat trees come with baskets, so the edge would keep the cat safer, plus the material is easy to hold if he does slip.
• Use a deterrent spray along the balcony edge.
• Virolino's suggestion is totally valid too - maybe the cat realizes you'll come pet and hold him when hangs out by the balcony. As hard as it might be, cat's are smart and resourceful and rather dramatic, so he might just acting. Try ignoring him when he's doing that, and give him lots of play and attention during other times.