Adding slopes against desert hills

I'm am building up a slope and grade for a narrow gauge line passing through Owenyo, California to Clovis, New Mexico.
I am building up the terrain with expanding foam from a can. After it is dry and hard, I will shape it with a sharp kitchen knife and slather it with fiber decor modeling material. These mountain sections must be removable so I lay down some cellophane wrap under the area I am working on so the foam will not adhere to the board.