Hi Mike, I'm in the process of the same mod (among many others). I'm apposed to cutting a bigger hole in the roof to house the highly rated Fantastic fan. That and from what I've read here, most people prefer the lowest setting (478CFM) or wish it could be set lower. There are two options I can find. One, as Donna suggested, is to use a computer fan. The largest available are 200mm (7.87"). They push out around 110 CFM and are focused on running quite (they're designed for computer gamers). Brands are Cooler Master Mega Flow or the Antec big boy. Both can be found on Amazon or ebay. I'm thinking I might cut a piece of plastic to frame it in then just lay it on top off the heavy duty screen in my boler
with a small hole to run the cable through. This way I can use the existing vent cover. Very cheap ($20-$40)mod with little work
The second idea is to use a solar
powered marine vent. The better ones run 1100 CFH = a meagre 18 CFM. These are meant to prevent moisture build up or a hot enclosed RV (or boat). I've wondered if anyone else here has thought of these for mould control. There's the advantage of not running wire. The disadvantage is not getting a breeze effect or perhaps just not enough air flow period. The fit is also a concern. Most are less than 9" round which would require some flanging and water sealing. They may extend down into the cabin an inch or so.
They are pricier.
Amazon.com: Sunforce 81300 Stainless Steel Solar Vent: Automotive
The only one I could find that was square and actually bigger than 9". Barely.
Amazon.com: Square Solar Powered Ventilator Water Resistant with Rechargeable Battery: Sports & Outdoors
I was hoping to get this done and post it w/pics so I could give back something more than my opinion to the FGRV community but if you can beat me to it, go for it.
I'm heading to Burning Man this year and will be siting in the desert for a week. I will need some sort of fan. Especially if I'm away from a closed up boler
for hours. Any feed back or changes to these ideas are welcome