I recommend the Silverstone Tek FF123B Ultra fine 120mm magnetic fan filter. I use 3 FF123B magnetic filters with my current computer. One covers the grill on the floor of the case, another covers an opening under the external drive bays (where you put DVD drives) and the third covers the grill on the left panel. Together they prevent a lot of dust from entering the computer. Since they’re magnetic they stick to the case and they stay where I put them. To clean them I just rinse them and let them air dry.
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