Desired technique for customising navmesh on a terrain

Hello again!

I have a terrain, parts of which are navigateable and others should not be. In my specific case, I have a road with crossings. This means I have to manually choose which areas are not allowed for my characters to move into.

To solve this, I used several navmesh modifier volume scripts and defined them in the road areas. It works fine - however this is a demo scene with a road that runs from left to right, perfectly straight. The volumes are only cuboid and cannot even be rotated.

Is there a better option available? Can I mark out a pseudo-2D region that sits on top of a terrain for a similar purpose?

Thank you, again!