I'd look for cases with a 120mm intake fan and fit the home AC filter material to it.
A very slight positive pressure keeps the opticals clean, floopy if you have one or card reader.
The Antec SLK-3000B has a 120 intake but it also has a door. *shrugs* Good news, it's about $50 at Amazon or Froogle.
My two rigs are in a garage office and stay pretty clean
but they are 24" off the floor. You can not believe the difference 8" makes
...Galvanized