Lightweight chipboard (which makes it a little flexible — good to wrap around curves/corners when stenciling).
If you're planning to use the stencil a lot, 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.