3mm plywood or plastic sheet
2 small neodymium magnets (those very small but very strong ones...)
threaded bar or nuts and bolts
battery or power supply
Glue the two magnets to the centre of the fan, slightly apart. Make sure it is the piece of the fan that spins. Make sure that one battery has the North pole facing upwards and that the other has the South pole facing upwards. If you don't then your magnetic bead will just sit twitching in the bottom of the flask.
Put the bolts or threaded bar through the holes in the corners of the fan casing to provide the support and fastening for the stage.
Measure and cut the wood or plastic so that it fits above the fan and will locate on the four bolts. You may want to add a small 'lug' on two sides for fixing the switch and the battery.
Fix the stage onto the fan.
You can now make your circuit either on breadboard, project board, or even etch your own PCB. The positive lead from your power source connects to terminal 1 of your pot, the fan connects to terminals 2 (the swiper) and 3 which are wired together. The negative lead from your power source connects to the negative lead on your fan. Switches and LEDs are always worth adding too - switches beacuse they are practical and LEDs because they just look good and tell you when it is switched on.
Mount your circuit board to your your stirrer and you're done.