waitwait why would you duplicate that much stuff if you can just delete the map.
I've done it before and it works, it acts as though you've built way up in the sky.
You just have the make sure water is low or else it won't work, since I don't know if it's works in v21 but slate maps had water for the texture or something, but you can go into mission editor and delete the ground.
granted you never able to hammer something or wand it or everything falls.. you could move the map down some.