I recently decided to do the same thing but decided to use cast iron for it's properties. Of course I didn't have a piece big enough and wound up on line.
No joke, a 3 inch square hunk of cast iron wound up costing me $72 delivered to my door.
In hindsight, if I were to do it again I would have used the piece of structural Aluminum I have sitting on my bench. It is very hard for Aluminum and would have worked just fine.
Good luck no matter what you wind up using, I just thought I would share a small example of my stupidity....
Phil