Mounting it in a cupola is an interesting idea.  I just wonder if you are going to pull so much attic air through the roof vents and other gaps and cracks that you won't get enough flow up through the ceiling grill?

One nice thing about the cupola mount is that it would be quieter.  They can be on the noisy side at high speed -- two speeds is very helpful to cut the noise down on low speed and still get a lot of ventilation.

Even if you mount the fan at ceiling level, it will cool the attic by blowing the house air through it, which is likely to be a lot cooler than attic air.