This schematic was built in a few hours so it isn't exactly perfect (it was also my first attempt at mcedit). That said, feel free to edit the shape and build upon the island as you see fit. I designed this floating island with the idea that any form of minigame could be built around it (be it uhc, hungergames or some wierd form of walls where you divide the map into quarters.)

I hope you enjoy the island :P