If you have a Thompson Center Hawken, everything but the patch box is available from TOTW, The Hawken Shop, and a couple of other places.
I know I have seen steel patch boxes in the past for this rifle, but I have looked high and low for one and they appear to be unicorns nowadays.
I would not try to make an exact copy in steel, I'd make it just a tad larger for a tight inlet. The Hawken Shop sells a larger "pineapple" style box that could be substituted, too.
I think it's better to darken the stock brass in this case and shoot the gun.