Is there a method to create a mesh by hand?
Yes, you can use the navmesh graph.
Thank you! Thank you! Thank you!
I also found a method using ProBuilder. I’ll make the polys using it, assign it properly and then disable the mesh when I’m ready to play my scene. It does not create a hard mesh (it’s still influenced by the navmesh settings) but it’s helpful when you can’t tune things properly.