Try cutting an air conditioner filter for the front grill fan. I put in filters in my cases. I use this:
http://www.amazon.com/Duck-1285234-2...ds=duck+filter
Thanks, was looking for this type of filter but was not able to find this kind of filter at home depot/lowes. Ended up using electrostatic vent filters from home depot. More restrictive and not the size I wanted, but has worked well and was something like 10 for $4.