I wrote earlier about using pre-stretched art canvas from an art supply store as a baseboard. What if you want to take a frame-and-profile, or "cookie cutter," approach to constructing your base and terrain, and if, like me, you don't like cutting and assembling wood?
How about starting with that same sturdy pre-stretched art canvas, and just unstapling and removing the canvas? Instant wood frame. To this you can hot-glue foam board or styrofoam profiles for the front, back, and sides, as well as for your track bed and any flat areas. Then just add your chicken wire or cardboard webbing, plaster cloth or glueshell, etc.
Save the canvas of course, you never know how you may be able to use it later!