When I build my stir plates, it's important to get the magnets sized correctly. The drive magnet that you mount to the fan hub should be the same dimensions as the stir bar. If you are really using a 1/8" stir bar, that going to be hard to find drive magnets of that size. I use a different approach on the stir plates I build. I use two small neodymium button magnets on the fan hub, space them apart so that the distance between centers of the magnets is the same as the length of the stir bar. That seems to be the best geometry for me.

Take a look at my stir plate web page for more details.
http://www.stirstarters.com