I do pretty much all of it with a flat chisel. Even the concave areas, if any.
Other than Berks county rifles, which often have some degree of concavity around the lock panels, and some of the Bethlehem/Christian's Spring rifles, which sometimes have a little concavity on either side of the breech tang, it's generally pretty flat most of the way around on most flintlock rifle styles, I think. I see so many people want to take a round file and hog out this horrible little trough all the way around the lock panels...it's so wrong.