After reading this thread last week, I immediately went to Amazon and purchased the Thermaltake usb fan unit and I couldn't be happier.
I completely removed the fan shroud and the speed control knob. The fan was attached to the base of the box using double sided sticky tape. I used the box that my RAZR Maxx came in for the case. I punched a hole in the box for the speed control knob to stick through and just used a bit of crazy glue on the back side to hold everything in place. I punched another hole out the back for the power cable.
The washer the hard drive magnet is attached to is held to the fan by double sided sticky tape. I used it with a 1" stir bar to whip up my last 1L batch of starter and it worked great!
Even on low, this DYI stir plate has no problem creating a vortex.
Thanks for the tip!