Remote NavMesh?

I have looked around, but I could not find anything specific enough to help in my case…

I am creating a twin stick space shooter and I need a NavMesh for the AI, but all of the AI will be “floating” above - not touching the NavMesh. All of the actors are at z=0.
Without a static mesh colliding, the NavMesh draws nothing.
The problem is when I use a static mesh for the NavMesh it collides with the AI and anything else with collision at z=0.
I tried to creating and using custom collision channels, but it will either collide with all objects or none of them.
How can I have a NavMesh that assumes everywhere is navigable except for the objects at z=0?**

I suppose I could just have everything sit on the NavMesh’s static mesh for the surface, but wouldn’t that add friction?

So I have been thinking a lot about this and I believe if I make the NavMesh tall enough to fit all of the actors above the “Surface” Static Mesh actor and have them all inside the NavMesh, I should get the right effect. Right?