Warning to all who follow:
Really Strong Magnets can cause more trouble than they would seem to be worth. Hard to believe but appearantly true.
This is good advice, though I would say that appropriate voltage is also really important. I recently made this and I discovered there is a pretty intricate interaction here between the electromagnets internal to the DC motor, the magnetic strength of mounted magnet , the stir bar magnetic strength, the size of both the stir bar and the mounted fan magnet, the voltage across the fan, the distance from the stir bar to the mounted magnet, and how well everything is centered.
Achieving the optimal balance is really quite difficult and can only be realized through some trial and error.
For instance, I have a 9V, 1A wall transformer powering a 3.5" computer case fan that is spinning two "fat" hard drive magnets (there are typically two sizes of hard drive magnets, fat and thin) stuck together.
It should be noted that the mounted magnet will slow down the fan considerably as its magnetic field interferes with the DC motor electromagnet. Too strong a magnet and you could prevent the fan from moving entirely. This can be absolved by adding a thicker spacer between the fan and the magnet, but I found that it was unnecessary in this case.
Mounting the fan too close to the stir bar creates too great a strain on the motor and it cannot move the stir bar fast enough to pull the vortex all the way down. Mounting the fan too far from the stir bar creates too weak an attraction between the magnets and the stir bar gets kicked off center.
It's tedious trying to find the perfect balance. I'm still working that out. I can easily get a full vortex on 1000 mL of water in a 2000 mL flask, but it splashes a lot and would end up coating the sides with a lot of sticky wort after 24 hours. I can't quite pull the vortex all the way to the bottom of a 1000 mL flask with 1000 mL of water with my current setup. I'm thinking probably voltage will do the trick as the magnets have proven themselves strong enough to hold the spin bar in place.