The Aeolus cooling pad features just a single fan, but it's an 8.66-inch diameter monster, capable of reducing temps by 22 degrees Celsius (39.6 degrees Fahrenheit) ...

The supplies to make one filter box cost me $127: $30 for the 20-inch box fan, about $90 for a four-pack of Merv-13 filters (20 x 20 x 1), and $7 for a big roll of duct tape.