Choosing a fan
At the heart of any air purifier is the fan. A small square shaped desk fan is best suited for this project. It is possible to use any size fan, however, as long as the fan is contained in a square or rectangular frame.
Construct the housing
Cut 4 pieces of 1/4-inch plywood to the outer dimensions of the fan you chose. Glue these pieces together at 90-degree angles to form the housing unit for your air purifier. Allow 24 hours for the glue to completely dry.
Purchase a HEPA certified air filter, and using a utility knife, cut it to the outside dimensions of the housing unit you just constructed. Using construction glue, attach the HEPA filter to the housing unit, which will form an airtight seal around the edges. Allow 24 hours for the glue to completely dry.
Spread a bead of silicone based epoxy around the back edges of the fan casing. Gently slip the housing unit onto the fan casing, ensuring that there is enough epoxy to fill any gaps between the fan and the housing unit. Allow at least 24 hours for the epoxy to cure.
Air is to be drawn through the housing unit from the fan, rather than pushed into it.
Turn the fan on, and listen for any abnormal noises. Aside from the sound of the fan running, the unit should be quiet. There should be only be a slight vibration when you place your hand on it. If the air purifier is emitting noises or vibrating excessively, check to ensure that the epoxy seal between the fan casing and housing unit is completely cured and solid, with no gaps.
Run your air purifier for 15 minutes, then check the temperature of the fan. If the fan casing is hot to the touch, ensure that nothing is obstructing the fan or in the path of the fan blades.