How to create a dynamic texture below meshes?

Hello, I am pretty new to Unity and I am currently developing a strategic game. Now if you take a look at this mockup you see that our artist put these lines below the hexagon-island.

I am looking for a way to create those, dynamically of course since the islands can have any shape you can build with hexagons.

I was unsuccessful so far to even just find something similar. I tried applying a plane-mesh to every tile but I think that is a very messy approach and in addition does not allow me the black outline.

Another idea was to figure out a way to detect the island as one piece and apply some kind of "outline" to it that looks like this. Can shaders do this?

Maybe someone here knows a better way or even something that has already been done. Much appreciated!