Last year I had a challenge with a computer that worked near a quarry. The huge ammount of dust was a constant problem and the computer had to be cleaned all the time. I tried all sorts of dust filters but nothing would stop the dust from going in. Eventually i started improvising, but the things i got that could hold the dust off, would reduce the air flow and the place was very hot so that was not an option. Also the owner didn't want to buy a water cooler.
I tried foam, a sponge, a sock, a piece of an old shirt and three metal filters put together, but it would either suffocate the computer or let dust though. There was no inbetween. After almost a dozen attempts, i got the perfect solution: a pantyhose.
They let enough air through but won't allow even very small particles of dust. Unfortunatelly i didn't manage to take a picture, but a few months ago did a service in an office that had two computers standing righ by the door where heavy trucks would drive by and a lot of dust got in. So I cleaned both computers and installed the pantyhose filter in one of them. Then, when i came back there i took the pictures showing the difference between the one with the filter and the one with no filter.
On the parts, there is no dust at all! It's like i left it three months ago. There is a little bit of dust at the bottom of the case, which i believe is entering through the opening on the left (i guessed the air pressure would prevent the air from going in, but on hindsight that was a supid guess).