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.
Step 3: Duct Tape Filter in Place
And no custom filters to go obsolete on you!
The squirrel cage fan is quiet on low speed, so it doesn't flip out the cats.
Hope you (and your cats) like it.
(OK, the cats couldn't care less...)