SInce you can't drill a square hole through steel, I made my hardy hole round. (5/8") I then tightened a bolt and nut through the overhang, bolt on top, and welded the nut in place on the underside.
I made a number of hardy tools, such as a cut-off, welding them to the head of different bolts and oriented across the anvil face. Loosen to change them and when you tighten the next one up, it is oriented properly.
They are nothing like as quick to use as a square hardy, which can be removed and replaced in a regular anvil in a second, but they do the job.