The bic pen body provides not only a flat surface to mount the magnets to, but it's hollow as well so the A1302 can fit inside it. You could use pretty much any non-ferrous material for the input shaft as long as it's hollow or has a pocket of some kind bored in the center for the sensor to fit.
You can buy the Allegro A1302 in the US from Jameco:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_1718743_-1They're $1.29 in single quantities.
g.
