Scribe

scribe.scad and scribe.stl

The scribe can be used to mark inside or outside corner contours.  An example of an inside corner would be fitting a counter top to an undulating wall.  Ride the cylinders, oriented up, against the wall with a pencil inserted into a hole chosen according to the amount of counter you need to remove.  Examples of outside corners would be a table top or the front of a guitar.  Ride the cylinders, oriented down, along the edge to mark a contour a consistent distance from the edge.  The .stl provides one version of the tool, but the included .scad file may be modified as needed.