Ceiling vent for ceiling that follows roof line

[thool]

I have a situation where I'd like to install a bathroom vent fan on a ceiling that is drywalled and follows the slope of the roof. There are 2x6 rafters with insulation, and no way to vent down the slope or up the slope. The most direct way I see is to cut a hole in the roof sheathing, install a 90, and then link up the fan. I imagine a low profile unit would be needed.

The location already has a switched light fixture attached to a box. My plan is to use that power to run the vent's fan and light together.

I've never done this before and am not sure how a blind install like this should go. Any pointers?

 

[buildyourown]

They make wall fans that are sized to fit in 2x4 walls. Panasonic makes one with a regular blade fan that could work if the pitch was steep enough. Too shallow and rain will come in through the opening.
I have used another wall unit that had a squirrel cage blower that exhausted out the side. It then stuck that into a 90 out the wall. Not the most effective fan, but you work with the space you have.