Make drag selection follow terrain normals?

Hey!

Im currently working on city/settlement/village builder and have a question.

Im wondering how to make a drag selection area (to select units or resources) to follow the terrain normals?
Currenlty im just making a box like drag but that one goes trough terrain. and does not work to well.

Added a image to show what i currently have,
As you can see this is just a box that resizes but goes trough the terrain on some sports depending on what height i started dragging it at etc.

Maybe look into Projectors? It could achieve what you’re looking for.