Shotgun flashlight clamps are usually made of plastic or aluminum. They cost about $60-80. Magnetic flashlight clamps are little cheaper, about $30-40. But what if I say that you can buy shotgun flashlight clamp for just five bucks? And this is not a joke.
My friend gave me inexpensive Chinese bicycle clamp which is made of strap and plastic. He told me that it can be easily used as shotgun flashlight mount.