Tissue for an air filter?
A little background, I have a foam-like air filter on the from on my computer case and it works fantastically and I am wanting to replicate the air filter on the side panel. The side panel of my case has two meshed windows, one for the CPU where the CPU
Your best bet is foam. I use the thin replacement filters for air conditioners. I can get them cheap at Home Depot. just tape a piece over your intakes and your all set.You can buy ready-made filters with grills at CrazyPC.com, or make them yourself.

