LOL... that Man Law commercial was pretty funny with the best line coming from Jimmy Johnson.
Anyways, just go to your local art store or hardware store and look for thick cardboard. There's a name for that type of material, but I've no clue what it's called. Regular old posterboard won't work so well, because it's flimsy and liable to fall apart if it gets wet or something. Styrofoam will slowly disintegrate and can easily snap. But I guess you can glue a bunch of regular posterboard together, to make it stiff and thick. (All you guys with dirty minds, try to ignore that, yea? LOL).