ZBrushCentral

Sculpt within lines

Hello, how to draw an object that fits exactly within the lines of a reference drawing. What it is, is a sign that has a cut out for the zbrush drawing/object to fit into exactly. How do you draw in zbrush where the zbrush object fits the cutout? Like snapping to an outline???
Any suggestions, I have always used zbrush with no restrictions.
Thanks in advance, George