Probably your best bet is to use particle board shelving that you find at either Home Depot or any other home center. It is about 11 1/4" x 96" and is cheaper than wood. It comes in pre-finished in white or a wood look and you can get it with a "bullnose" edge (one edge finished). I don't think you would want anything wider since that should be enough for the track and plenty of scenery. You can also find a variety of brackets for support, from plain to decorative to wood.