Meet the Magswitch MagJig: a cool magnetic clamp that you can activate with the turn of a knob. You simply have to drill a hole and put the jib in it for downward pressure clamping. Once you are done working, you simply have to turn the knob all the way off for release.
The MagJig 95 can hold flat and round steel items. It doesn’t require any other power source to work. These are made with 60 to 235lbs holding force.