Laser cutter magnetic hold down clamp

This is a magnetic hold down for in my laser cutter to pull slightly warped material flat.

Requires a 1/4 nut an bold, a couple washers, and a spring. I just used parts I had laying around. It will also need a fairly powerful magnet. Anything will work, but it needs to be strong enough to hold the clamp down. The bottom of the magnet pocket is only 1mm thick.