Dust filters on all air intakes.
I like that
black foam air filter stuff (used in air ducts, air conditioners, microphone windscreens, high-end speakers, etc). It's cheap and common and easy to cut to size. It can be vacuumed, blasted with compressed air, rinsed under running water, thrown into a dishwasher or laundry machine, immersed in soapy detergent stuff for squishy spongy scrubbing. It's even handy for wiping up dust from other surfaces, lol. And it's black, which always looks cool in gaming hardware. Win-win-win!
A positive-pressure chassis needs these filters on the fan intakes. A negative-pressure chassis needs these filter pads stuffed or lined into all the little vents and holes and gaps and crevices where inflow occurs - especially any bottom fan vents and "floor" level air holes.
Small upside is that this stuff slightly muffles noise. Mostly fan noise, but some other things inside the chassis can make noise too. And it's wonderful at completely neutralizing "rattling" or vibrational noises caused by the chassis panels themselves. It's not full soundproofing "silent PC" stuff, but it sure helps cut down noise when put in front of the fans making the noise (especially when those fans are basically all directly facing the user).
Small downside is that this stuff slightly reduces airflow pressure. Not volume, pressure. So unsuitable for rads (which need strong fans and even push-pull fans to move air across them) ... although intake across rads is always suboptimal and exhaust across rads shouldn't want to capture dust inside the chassis, lol.
Increasing thickness increases these effects.
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