2″ x 5/16″ or 40mm x 8mm gauge plate would do but you will have to buy longer. Try Coventry grinders who’s price for 18″ or 500mm is cheaper than the ME suppliers who do 9″ or 250mm lengths.
If the hardening is only for surface wear eg rubbing then consider case hardening mild steel, the carbon steels are for through hardening if you want say a cutting edge