A recent article in "American Machinist" reminded me of something I made
twenty years ago for a friend.  The body of the tool is a
through-drilled cylinder with one quarter of its body accurately milled
away.

Insert the punch, made from hardened drill rod, into the hole, align the
edges of the cutaway portion with your scribe lines and voila (not viola
- a musical instrument), the punch mark is on the intersection of the lines.