For one as wide as a shotgun rib, I simply make one from a used up dull file. Shape it like a wood chisel. Put a 45 or 50* face on it and a very slight heel,,maybe 5* or so just to strengthen the edge..
Keep the tool a handy length like 3 inches and put a comfortable handle onto it that you can hold in the palm of your hand to keep control and put pressure/force directly to it.
Regular hand graver handles if you any to choose from,,if not just make something up that mimics that style. Doesn't have to be fancy.
