Spawn Objects on Walkable Surfaces

So I have a few enemies that use the Navmesh system to find their way to the player. Now I want my enemies to spawn on walkable surface(the blue part) of the Navmesh. I want the spawn coordinates to be random but inside the walkable surface as I don’t want them inside the walls over my surface.

Is there some way that I can get walkable surface and limit the coordinates only inside the surface. What I
could do is that Shine a ray down on the arena and if its the walkable surface then that’s where the enemy will spawn, but I still need a way to make sure its the walkable surface.

So i just got the answer. I used the above method but instead of using Navmesh to determine spawn coordinates, what I used was tags. If the ray hit an obstacle with a certain tag it would run the process again and again until there was a suitable place to spawn and spawn the enemy.