Political signs make great cutting stock for CO2 lasers, and starting Wednesday you can get a boatload of them for free!

The official term for the stiff material is "corrogated plastic"(*), and it's basically corrugated cardbord using polypropylene instead of paper. It's light, stiff, strong, and waterproof.

Here's a desk organizer made from it. The pieces were lasercut and then hot-glued together. (Then glued to a wooden base.)

Here's a closeup showing the corrugation style of the board.

Some older types might be PVC, and you shouldn't cut these on the laser, so be sure to perform a flame test on anything you find before you start cutting. Modern signs should be polypropylene, which is fine to cut.

(*) Some trade names:  Cartonplast®, Polyflute, Coroplast, FlutePlast, IntePro, Proplex, Correx, Twinplast, Corriflute or Corflute