Been wanting to make one for stuff like this, has many other uses too, like on welding tables with square holes cut or cast in them such as Acorn. I plan on making several more to use on my bigger table, gotta make some sleeved holes in it first tho.
Nothing more than a Vise Grip 11R (never use cheap ones, they are crap) with a 1" square piece of stock welded to where the stationary jaw was. I added an extension to the moving jaw, just a small piece of scrap. The force of pressure against the jaw simply jams it in the hardy hole, locking it in place. It can be made infinitely adjustable too, just make the bar for the hardy hole longer.
I'm in the process of making a montage for some address numbers, needed something to hold smaller stuff like the leaf pictured down on the anvil so I can detail it without it moving all over the place.
I'm by far NO expert blacksmith or ornamental iron artist, but I like to mess around with it when I get time.