Most boxes are made of HDPE, or high-density polyethylene. Planet Snow uses it for their top sheets. It slides very easily, is more flexible than PVC, and doesn't break too easily, although wide sheets of it (for example, those covering a butter box) can crack and even shatter if hit hard enough. Thats why I always yell at people who 'stomp' onto a box. You could break the HDPE very easily. So dont stomp on the box, stomp your landing...