um… I’m throwing out some wild guesses here… I’m no pro, just a bun lover 
The space between the rails that I can see can be blocked off with chicken wires? or bug screen? or horizontal wires that limit the space further so that the best your bun could do is stick the nose out to investigate. Nothing wider though – I’ve seen Thumper squeeze himself into impossible spaces (well they
were impossible till I saw him rummaging around in them. Like our couch which is flat against the wall except for a bit at the feet/bottom). You don’t want your bun to be able to squeeze through any more than the nose, because with their innate curiosity it may encourage the bun to go further.
I don’t advise plastic blocking just because you don’t want your bun to be ingesting that, and if the plastic is chipped away at day by day it could provide an opening that you’re trying to avoid.
Also block off the part with no rail with something heavy – something that your bun can’t push away or gnaw through or squeeze through.
I have to say I’m more worried about the cats, though. Man they are sneaky! And they’re predators by nature – a bun running around maybe very tempting. Also, you will still need to keep an eye on your bun for birds of prey – if it’s a balcony and there’s no roof, all it takes is one swoop.
Overall, though, I’d say your bun doesn’t miss being outside one bit. If she has a large cage and is free range most of the time, she’s probably happy and comfy where she is. Domestic rabbits are meant to be inside, so chances are if you have enough toys and stimulation, they’re fine!