This is a way to instantly create a spleef arena without downloads in vanilla Minecraft!
1.Go to Create new world under single player
2.Go to more world options
3. Click on default until it shows superflat and then click customize
4. Click on Use Presets
5. In the text box, delete everything and type this EXACTLY: 1x1,5x11,2x0,1x80
6. Click on Use Preset, then done, then create the world and it will be a spleef arena as large as the world itself.
Use /gamerule keepInventory true to let people keep their shovels, and spawnpoint everyone to a mutual area that no one will destroy. Then... Have fun!