I use the 4' X 8' green/blue rigid foam boards from Lowes. They come in both 3/4" and 1" thick variations. I use liquid nails to glue it to the plywood base and place anything heavy over it until it cures (tools, books, etc.). The boards can be cut and trimmed with a variety of tools but I use a long and very sharp commercial butcher knife.
After it's installed, I mask off any roads with duct tape and paint the foam board with an earth brown latex paint from Lowes and sprinkle it with yellow and green fine turf as I go along while the paint is still wet, so it adheres to the paint. That gives you an instant ground cover for the whole layout. I then go back and peel off the duct tape and paint the roads a flat black or grey concrete color. Any seams can be covered with duct tape before painting and painted over. You can use a flexible vinyl type joint compound to do the seams, but I find it unneccesary. Once everything is dry, you can then lay your track wherever you like and install your trees, poles and accessories.
Note - I have subsequently sprinkled fine black cinder over the turf in the yard for a more prototypical look.
