Navigation mesh volume requirements

I have a level that is basically a solar system and ships are supposed to be able to move around the 2D plane. I’d like to use the navigation mesh to make my life easier but from what I can tell it requires a surface to actually create a navigable region. My thought right now is to basically create an invisible plane just below the plane I want things to be able to move about, though I get the feeling that I would need to play around a bit with what it collides with so that the static meshes that are the planets can still move around it. Has anyone else had any experience with this kind of setup? Or have any suggestions for approaching this situation.