So I made my own air purifier out of a cheap fan and a cheap furnace filter. Here's how...
Step 1: What You Need
* Squirrel-Cage Fan - $22
** Air Filter - $8
* Standard rotary fans don't work as well. Plus, it's harder to cut a circular piece of filter.
** You pick how much filtering power you want by choosing the quality of the air filter. You can get different MERV ratings. Spun glass filters don't work; forget those. Pick a filter height that matches the height of the air flow openings in your fan.
Step 2: Open up furnace filter, cut to size.
Keep the wire mesh frame; it's too hard to remove.
Stretch-out the pleated filter so it is flat. I used the edge of a table to straighten each pleat.
Cut a piece that completely covers the back air flow opening on the fan. Use old scissors - they will cut through the wire mesh but will damage them slightly. Using wire cutters on the mesh only for a first-pass will work too. Then use scissors to cut the filter itself.