And if you try to create one you’re gonna get a reality check really quick when you allegedly threaten the sovereignty of some nation state like those seasteading people did.
How about just about to make your own island in international water?
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.
What about a really, really big oil rig?
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.