Here is my stir plate build I did a couple years ago...

Stuff you will need:

1) a project box, I used a 5x7x3 Radio shack plastic box or similar shaped box you have sitting around.

2) a computer fan, snag as many from old computers as you can... dead useful for making things.

3) 25 ohm 3 watt potentiometer, got mine from Radio shack when I picked up my project box, got a knob for it too.

4) a single pole single throw switch to turn it on and off.

5) power supply, 12v wall wort works pretty good, mine came from an old cordless drill charger.

6) 3/4" PVC coupler, may need to be trimmed, I used a miter box to cut mine. The coupler is used as a spacer to raise the magnets away from the fan where they could cause interference in performance, and also to be able to set the airgap between the magnets and the box lid.

7) 5 minute epoxy

8) Mounting hardware for the fan, and adhesive rubber feet for the box to keep it from moving around when in use.

9) Name tag magnet bar, it has two disk magnets in a plastic frame, if you have ever worked retail you likely have seen them.

10) Erlenmeyer flask and a stirbar.

The assembly:

Place the computer fan in the center of the bottom of the project box and mark the hole locations to mount it, also mark locations on the front of the box for the switch and potentiometer, mark the back corner for the power cord to go through and also mark a few vent holes along the back of the box.

Using a unibit or standard bit set drill your holes, the mounting holes are rather small, as is the hole for the power cord, the holes for the switch, pot, and vent holes are a good bit larger.

mount the fan, the switch and pot, snip the end of the wall wort off so it can be fed through the small hole you drilled.

With enough wire pulled through to reach the pot and switch, tie a knot in the power wire to keep it from pulling back through, I further secured my wire with a dab of hot glue.

Before soldering any connections I would twist the wires in place to be sure it will work, attach the negitive lead of the power supply to one lug of the switch, attch the positive lead to the outside lug of the pot. connect the fan negitive lead to the other lug of the switch, and the positive lead to the center lug of the pot, plug in your power supply and check that everything works, the switch should kill the fan, and the pot should vary speed, from slowest or stopped at full counter clockwise, to full speed when full clockwise.

If the pot works backwards, move the positive lead to the opposite side outer lug.

With everything working properly, solder the wires to the lugs.

Time to mount the magnets and coupler!

Test fit the coupler on top of the center of the fan, and the magnet gizmo on top of that, check for a small air gap between the lid and magnets, I placed the lid across the box just to the magnets and found it would hit so I trimmed a tiny bit off the coupler with a miter box and hand saw, I set my clearance to about 1/16" below the lid.

Now that everything is trimmed:

Working fast, epoxy the coupler to the center of the fan using 5 minute epoxy, also epoxy the magnet thing to the top of the coupler, working fast I spun the fan a few times while adjusting the coupler to be centered on the fan, and centered the magnets across the top, with everything positioned correctly let the epoxy harden for a half hour or so.

Screw the lid down and turn the stirplate back on to verify there is no contact issues, fill the flask with water, drop in the stirbar and center it on the stir plate and let er rip!!

With everything working correctly you can now grow a huge starter or harvest the smallest dregs of yeast from your favorite craft beer!

***Here are pics of the assembly to clarify the steps above***