Use as a stencil, cut it apart or use whole as a background.
Lightweight chipboard (which makes it a little flexible — good to wrap around curves/corners when stenciling).
If you're planning to use it as a stencil, I would use spray shellac to seal the stencil before you get paint on it (an oil-based varnish, rather than a water-based acrylic spray will last longer). Spray a few layers and let dry. Flip it over and do again.
Design Team Member Caroline Ouzts-Hay made A Parade of Masks (see the wide photo below). More info in the blog and/or see the supply list.