Are you able to change the Terrain type in Forge? The current state of the editor is pathetic, while it has more options then Halo Reaches forge, the maps are just to small. They need to release an update ASAP or we will never see great maps like Hemmorage in Big Team Battle again.
Still, couldn't you just use the Valhalla map and remove everything and start over? It is not as big as Hemmorage though.