Even if you could cut them, the cutting would probably result in them being distorted. They are a lot thinner than flat telescope mirrors, so keeping them perfectly flat in a telescope would be difficult, if not impossible. And because something looks reflective, doesn't mean it is reflective enough to use efficiently in a telescope. Those little suckers are expensive for a reason. But I'm no expert. It might work.
I would love to see someone mold a plastic telescope mirror then overcoat it with metal. If that could be done, big telescopes could be made cheap enough for the masses.