This is a refridgerator magnet that looks like a jumbo push pin.

Download pushpin.scad from http://www.thingiverse.com/thing:7911 and pushpinMagnet.scad and generate the push pin magnet as is or change the parameters (it's parametric!) to get the size you want.

Sized for a standard craft store disc magnet with 9mm radius and 5mm height.
Print the push pin and glue the magnet into the hole.