Why do you need the threaded rod and the spring? If you remove the spring loaded rod and connect the ground wire directly to the steel flat bar you'll get a better ground. Those neodymium magnets are pressed hard against your workpiece (much harder than the spring loaded threaded rod) and they're good electrical conductors. I think the threaded rod and spring is holding you back.

I have a magnetic ground that's just a mag base with a ground wire hard bolted to it. It works great.