I used that idea, but didnt want to spend $30 or whatever on that one filter, so this is what I did
The only thing I bought besides the air purifier was the activated carbon and the white mesh stuff, total of like $6.50
It works great and I leave it on 2, you can feel the air moving around the room.
littleindiangirl, the large dust particles will never make it to the hepa filter! especially with my setup lol