@OP:
Yea, even I have the CM690, and stay in South Delhi. Dust just gets to the system. Even the metal mesh is covered with dust. Really irritating.
There is a shop in Nehru place, which have these special blowers (small ones), which can be used to clean the computer. It is the 1st upper level on the entry from Paras Cinema. Right side wing. You can give that a try.
Yea, even I will try for the a/c vent material.