hi, im am new too the forum but your topic caught my eye. i am also doing a mt drift diorama with a 1/24 scale ae86 that will be done to look like my real one. first your base. i would use a wood base, most people use picture frames but the cardboard backing isnt very strong. next build up your mountain and road using pink or blue insulating house board. you can get this from any home hardware store. carve it to your desired shape. next cover the whole thing with a modeling putty/clay. aff the putty dries paint ground work in a earth color . add grass , plants and trees. next work on the road. you could make it look smooth , cracked or grity. as this is all done with the modeling clay with a thin layer for the road. next paint the road a asphalt color. use airbrush for skid marks road wear ect. to ad more details you can build guard rails and road signs out ove sheet and rod styrene. hope any of this helps
here are a few pics for a ref.