If you have a vent there already- you can purchase a Turbo MaxxAir powered vent.
Maxxair 00-965001 Turbo/Maxx 3 Speed Fan with Thermostat : Amazon.com : Automotive
This option lets you use your existing ceiling vent curb and crank.
The Turbo Maxx fits over your existing motor-less vent.
No un caulking or re-caulking required. The Turbo Maxx can be used even
in a rain storm.
If you want a Fantastic Fan-
Amazon.com: Fan-Tastic Vent 5000RBT Off White Vent with Reverse Thermostat: Automotive
You will need to remove your existing curb and install the Fantastic fan
and curb and crank open cover
AND you will still want a rain cover for it so you also want one of these-
MaxxAir 850 White Fan/Mate Rain Cover for High Powered Ceiling Fans : Amazon.com : Automotive
This cover allows you to use your fan even in pounding rain.
It's up to you. The Fantastic Fan option requires you to remove your
existing curb. The Turbo Maxx option fastens to your existing curb.