A very simple magnetic stirrer.

The design is flat and 3mm high, very easy to cut on a laser cutter, but can also be 3d printed.
Note the laser cut version is shown.

Required parts
120mm 12V DC fan
12V DC power supply, eg plug pack, old pc power supply etc
Magnets,
Super glue
Fan screws

Optional:
Extra wires
Switch
DC barrel socket

Basic instructions
Use super glue to fix the magnets to the centre hub of the fan - see image. 3 magnets per side works, but more would be stronger.
Install the fan (magnets up) into the base
If using, install and solder the switch and barrel socket.