I am recycling my CM 690 from my other build and some time ago, two of my stock fan's LEDs had burnt out so I had replaced it in the front. I decided to place a fan under the two optical drives (they are older and I painted them black but they are in great condition).
What I did was I used the 5.25 - 3.5 adapter tray that came with the CM 690 and attached the fan using 2 sided tape and zip ties. The result is a fan that I can slide in or out for cleaning and that locks into place using the toolless sytem in the CM 690.
I thought it was pretty clever actually!
I also used Black Fan Grill over my fans, but for some of them, I cut out each second circle to create a bit less drag where I did not need them for safety so much.
The last pick also shows that I cut out the plastic grills behind the mesh covers on the CM 690.