Make sure the hard drive magnet is perfectly centered on your fan. If the poles aren't centered the stir bar will try to spin out of wack and will always throw the bar.
You have a ridge in the center? THIS will keep it from throwing stir bar as well.
If your not boiling then, you don't need silicone O-rings, I use corny keg dip tube O-rings
Update: I got a 12mm stir bar and that seems to work perfect. Starter has been running for 15 hours now. It's a little noisy lol. Thanks for the help people