Besides a 'built-in' ridge at the top of the drawing, I'm planning on using some 'loose' hills, buildings, etc.
I            use 1/2" MDF (Medium Density Firber) boards for all of my game             boards. These can be obtained from most DIY stores and are  most commonly            found in 2x4' pieces, which is exactly what I use. You could  also use            2x2' sections (or whatever other dimensions you like - most  DIY centers            will even cut the boards for you), but I have found the  modeling options            to be better with the larger boards. Of course, they are  harder to store,            but I am lucky in the fact that I have a permanent room  devoted to my            hobby where I can stack such things.
MDF            is more resistant to warping than plywood which is one of the  reasons            I choose it. However, to make it even more resistant, and to  protect            the layer of foam that I intend to add on top, I frame all            of my boards with 3/4" square battens. These are carefully  glued            to the boards with Liquid Nails and then to make them stronger, I  flip            the boards over and secure the battens with 1" wood screws.  It's            a good idea to predrill holes slightly smaller than the  diameter of            the screws to reduce the chance of splitting the battens.
You            can always skip the framing step if you don't transport your  boards            and just glue a layer of foam to the board, but the wooden  frame makes            them extremely durable and adds a very professional look to  the finished            piece. I think the extra effort is worth the results.
I managed to frame out all six boards in an afternoon. To see the full details of my construction process and see where we'll end up you can check out the tutorial on my main site. I'll duplicate some of the info here and add new photos of this project as I go. If there are any questions, I'll be happy to answer them here! 
 








