The depth of most common house ventilation fans was my first thought. Most of them over 80cfm will be 10" to 12" thick. The heaters do pull about 1000w @120vac.
I'm using 2 120mm computer fans which are rated at about 100cfm each with a rheostat to control the speed. Venting them straight out the side with a shoot down to prevent water coming up it or in. I don't have a picture of the inside of this part but it is just two 4" holes and I will built a small enclosure on the opposite side with a filter for intake.
I have seem to have a fear of roof penetration also
