I was planning to build those for a while now I got a chance to build one. So let's start building.

Materials:

-PC Fan
-Neodymium Hard Disk Magnet
-SPST Switch
-Bunch of Round Magnets
-Alligator cables (for testing)
-Enclosure
-Wire

Tools:

-X-acto Knife
-Hot Glue Gun and Hot Glue
-Soldering Iron and Solder
-Rotary Tool
-Needle-nose Pliers

Step 1: Start Making Circuit

First of all I take old usb fan and try using it. But I discovered that it was too fast for this purpose. So I replaced it with brushless pc fan. It give perfect results.

Step 2: Preparing the Enclosure and Assemble Everything

I marked fan and switch on the box then I cut them with rotary tool. Then hot glued everything into the box and also hot glued bunch of magnets for water resistance . Next I painted the fan (Just looks good).