Your best bet is to use a piece of gauge plate of the correct thickness then harden it failing that a piece of machine hack saw blade though it will need to be annealed then hardened again. Going down in hardness and just still workable as is with hand tools is a DIY type circular saw blades, a lot of these blades are made from high carbon steel and can be heat treatable. I do find circular saw blade material very usable rather than paying a high price for a couple of inches of gauge plate etc.

The only lessons in my life I truly did learn from where the ones I paid for!