you could use pantyhose, or something similar. however, I would not use air filters, as they restrict airflow to an extent. It is much easier to simply open up your case every coulple of months an blow out the excess dust with a compressed can of air.
To further decrease dust you could have a positve air pressure flow in the case.
I was thinking the same thing. It would probably be easier (and cheaper) to make a filter than to buy one that size.