That cave is going to look killer! Are you thinking about doing a backlit background where you see through the cave (like a tunnel) to what appears to be the horizon, or are you going to black out the back of the cave to create a true cave effect? Either way will look good.
There was a member here that created a small cave or cove-like effect that looked really good, but I can't recall who it was. Maybe someone else can recall and share the link. It should be a great source of ideas.