I know a lot of DIY computer fan stirplates have been posted before, but everything I have found has said that you need a 12V fan from a desktop so I wanted to post my experience using a 5V laptop fan.
I wanted to make a computer fan stirplate, but didn't have an old desktop available. I did have an old toshiba laptop available. I tore it apart for the fan and hard drive and then used a minimum of other materials that I had around the house to make this. I have an arduino uno controller board that I have been using for a project at work and used it as a controller for this. Please ignore the ugly soldering job on the PCB. The case is from the hard drive and the plexiglass is from an old TV, I used a coping saw to cut an appropriate sized piece. I used a little JB weld (the grey stuff) to better attach the screws. The black cord in the photo is an external power source for the arduino, essentially I can upload the program and then let it run using the adaptor and not need the computer to be connected.
The Arduino program is very simple. I use digitalwrite for a very short burst to get it spinning, then use analogwrite on both leads essentially as PWM to run it a bit slower. If anyone is interested, I can post the code, though it is easily reproducible. I could also write it to be able to vary the speed using the serial monitor, but this works, so why bother.
This wouldn't be cost effective unless you already have a controller board handy, it usually costs 30 bucks or so for the board. I plan to use the arduino in a fermentation chamber later so it can to double duty.
In the photo there is a spacer between the magnet and the fan itself. This is because the magnet is strong enough disrupt the motor unless you give it a bit of distance. The spacer was also from the hard drive.
I'll admit, it is probably a little underpowered compared to the larger ones. I can get a very small vortex going, which should be more than adequate. A 1 inch stir bar is perfect for this, the larger ones don't really work. It may burn out at some point, and I'll update if it does. I picked up this 3 quart container for 4 bucks at Kroger. I will have to modify the top to fit an airlock, but is a cheap flat-bottomed container.