Texas Outlaw: If you do end up using plexiglass, ask for "Clear Acrylic". Plexiglass is the brand name for clear acrylic, so you can almost always save money by getting the generic stuff.
It *looks* like ThatDirtyKid used a colored acrylic, which is like clear acrylic/plexi, only with color. The reflective shine in the photo is what makes me think that.
The acrylic is a very good choice, I think, but it can be a *little* tougher to work with than a wood product. The best way to cut it is to do multple scores and then snap. This can sometimes yield less than desireable results. It would also be harder to get a rounded edge to the board. You could use a power sander to do that.
I think a masonite board would also be a good choice.
http://www.pearlpaint.com/shop~ocID~500 ... D~9863.htm
It is very thin, but is relatively solid with a good resistance to warping. It is harder to work with than plywood, though. So be aware of that if you use it.
All of this is theoretical -- I've done a lot of these kind of projects in the past so I have some experience, but I've never actually done this project. TDK hopefully can give more info.