Question Specify NavMash area for one terrain tile

Hey Com,

I have a terrain tile and I generated a NavMesh for it. However, the NavMesh is generated for the entire tile.
Is it possible to mark an area on a terrain tile for the NavMesh system to only bake the marked area?
E.g see picture below


UPDATE:
I played around and found a solution, that is rather dirty. I placed a few boxes, made them static and baked them as not walkable. In the next step, I added a new material and made all the boxes transparent. My question still persists, because this solution doesn't look professional to me.