There are various choices and it depends on how much work you want to do.
The easiest is the
VORTEX FANS. They screw right into the bottom. I have not used the Vortex II but the Vortex (I) is noisy enough that I would not use it in any place other than the bath and then not if it is right next to an etertainment area.
My pref. is the
FANTASTIC FAN. Quiet and plenty of air movement. Takes about 2 hours to remove the old vent, on the roof, and replace it witht e FF. You will need putty knife, screwdriver and
DICOR self-leveling lap. Think about putting the FF vent covers on while doing this. They work great for the FF to open up completely.
Good luck and have fun.