You might try making a bunch of small loops of cellotape (some people call this "scotch tape" or just "tape") and scattering them on top of the unsafe places. Cats hatehate getting anything stuck in their fur, and will be wary of any place they think this might happen. However, you should be there for the first time or two that they get stuck, so that you can remove the cellotape.
Since the cellotape can really upset the cat, and may be uncomfortable for them to have it removed, I don't recommend this as a general "cat-proofing" strategy. However, in this case the alternative (falling off the balcony) is far worse, so I think this technique is justified.