I started looking into fan filters and decided to try to make my own.
The fan filter shown below is from a company called Nexus (http://www.nexustek.nl/NXS-accessories.htm). They also sell various other interesting and useful products.
The thing that filters are supposed to do is prevent dirt from entering the case. Therefore, one can make his/her own. This one is made from cutting out a square from a black t-shirt. Once that is done, the square can be placed over the area and either screwed on, taped on, or whatever can be used.
Although, a t-shirt is used for this, various other material can be used. Looking around the hardware, clothing, or junkyard may provide better and/or cheaper filter material. It is only a matter of having the right tools for the job.