UNCLOS says that any artificial islands within the exclusive economic zone of a country are subject to that country. This means in practice that you'd have to build an island that's not on a continental shelf... good luck with that.
I don't know if you would even need a big one. There are definitely shipping container ships that have huge decks that are more than enough space. I think if you have a decent margin around a helicopter pad you might even be able fit the octagon, cameras, corners and judges.
