you can always use paper towels. but it might restrict airflow so i wouldn't use the heavy bounty types but maybe the store brand.
edit:
duh forgot about this stuff. http://www.geyservac.com/images/eureka 38333 foam filter A_thumb.jpg
they have this foam filter stuff that is thinner than that in the picture. they usually use it as a cushion padding in like steel breifcases. i acutally cut some and put it on my carburetor on my car.