I think we just need to give up, and return to the simpler non-electrical days.
Or, maybe not ....
The customer isn't about to spend the thousands of dollars necessary to buy a 'proper' rain resistant fan. This follows what's all too common in the residential market- there often are no commercially made products that fit the intended application.
Even a Bell box is questionable, as it will be mounted 'upside down," and careful use of the included gasket will only make problems worse, by trapping water inside the box.
That's been the saving grace of so many of the homeowner hacks we've seen over the years; the materials used leak the water out as fast as Nature puts water in.
So .... what would I do? I mean, it's always easier to find fault than it is to make things work. (We won't get into how "it works, doesn't it" is one of the primary excuses used for outrageous hack jobs, at least not right now).
As much as I like enclosed (Bell) boxes, I don't have much faith in their threads supporting a fan. I want the mounting screws going into either good ol' steel, or substantial wood. Thus, I'd start by having the framer construct a stout 'box' above the fan, a box that will contain a suitable fan box. As a fringe benefit, this box will remove the electrical connections from the weather.
I'd leave t at that. Inspect it before the fan is mounted. Leave the customer to his own devices. Start worrying about the exact fan that will be used, and before long you'll be looking at light fixtures and kitchen appliances. Someday, someone will ask just what / who allows cars into the garage.
