I won't repeat every word that smokinghole said above, but I could. HD magnets (3, stacked) only worked well with a 1/2" bar. And that DID work very well, and if I wasn't and endless doodler I would have kept that one. But I had a 2" bar that I really wanted to work, and I already had a large stack of small round rare earth magnets.
Rare earth magnets, 2 stacks, each stack contains 3. The stacks must have opposite polarity (one stack is north on top, the other south on top). Stacks separated by 1.5". Works perfectly with 2" bar.
To get it just right, I taped the fan assembly together, magnets and all, then taped to workbench. I then set my stir plate platform just above it with flask. This way I could keep changed the magnet position until I got it right. Then I hotglued the fan to the plate.