The stencil has some noteworthy design features:
The letters are designed to be held in place with adhesive tape.
The pie chart pointer is held in place by an adjustable-tension bolt and wing-nut.
A small cutout arrow indicates which portion of the statistic is being described by your text.
The perimeter of the pie chart is etched with 100 tick-marks, making it easy to adjust its percentage.
The letters (a new stencilized version of Trade Gothic) preserve correct character widths and are provided in proportion to letter frequency (e.g. ETAOIN SHRDLU).
Here are the stencil designs (Illustrator CS4 .PDF format):