Gary Fritz wrote:
It *is* possible to suck a lot of air through the thing -- I hooked
up a shopvac and it pulled more air than I wanted, along with a
thunderous rumble of turbulence. But that's a very noisy expensive
solution, and I haven't been able to find a regular fan that can do
it.

You can't get there from here.

The duct diameter is: 3.00 inches
Flow Rate 75 CFM
Velocity 1530 FPM
Pressure 14.7 PSI
Temperature 85 F
Duct length 20 FT
Duct Roughness 0.01 FT

At 1530 FPM, any fan is going to make a lot of noise.

How about opening up the ceiling over the projector and just venting it
into the attic?