Enco currently has the Mighty Mag magnetic base for $11.95.
One of those handy machine shop accessories, the Mighty Mag is a magnetic base that can be used to hold indicators and stop rods in a variety of configurations on just about any flat steel surface. The Mighty Mag bases have strong holding force (45 lbs.) and are made in the USA (be wary of knockoffs). There are a number of ways to mount objects in the Mighty Mag: three 1/4″-20 tapped holes, one #10-32 tapped hole, one 3/8″ reamed hole and the useful half vee groove hole that will hold rods of 1/8″ to 5/16″ diameter. The Enco model is #SR625-0440 in case the retail price gets returned when searching.
Mighty Mag [useenco.com]