I used a clear 90mm Antec TriCool double ball bearing fan (DBB for better life time). You don't notice the clear fan against the projector when its mounted on the ceiling. It has a speed selector that allows Lo/Med/Hi fan speeds. I used white twist ties pulled through the vent slats to secure the fan. I left a small space between the fan and the projector body to reduce vibration noise and to allow air to flow behind the fan. The more air is obstructed the more noise a fan will make.
I normally keep the fan on Lo (21cfm, 14db) but if its warm in the house I put it on Med (28cfm, 25db). The temperatures drop significantly on the bottom of the projector and inside the wind tunnel.
I didn't block the small vent on the bottom of the projector. I tried with and without a block but it didn't make a lot of difference temperature wise and I didn't want to starve the fan for air.
I decided not to blow air in from the intake vent because that vent is so small that most of the air blown by the fan is directed against the case and not in the vent. Also anything obstructing the vent, like the filter will tend to raise the fan noise as the air interacts with it. I wanted a fan to be as unobstructed as possible. The 90mm fan just fits over the exhaust vent nicely.