I can't spare any pantyhose, so I ordered some fan grilles and screens (foamy type). I made some blowholes, so the grilles also make the blowhole look better. I've also heard of people using cutouts of your standard air conditioner filter stuff. Also, you can decrease dust intake by having more positive airflow pressure than negative. Negative pressure means you'll be sucking in air/dust through any possible little crack in your case. You can also place an air/hepa filter near your computer, to cut down the dust in the room before it reaches your computer.
Chiz