Create air world

[NiLSPACE]

If you're willing to create a plugin for it you can just use this:

function OnChunkGenerating(a_World, a_ChunkX, a_ChunkZ, a_ChunkDesc)
	a_ChunkDesc:SetUseDefaultBiomes(false)
	a_ChunkDesc:SetUseDefaultComposition(false)
	a_ChunkDesc:SetUseDefaultFinish(false)
	a_ChunkDesc:SetUseDefaultHeight(false)
end

Otherwise the only possible thing I can think of it setting the ShapeGenerator to heightgen, the heightgen to Flat and the Flatheight to 0 or -1.