The BEST rafter mount systems April, 2017. List from Invisco.com. Note that QuietCool and Invisco are the only sources for over 6000 CFM. And Invisco now has a rooftop whole house fan at 8000 CFM.
Once you have sent your email, click here to see a trove of data on all the sources you can imagine: http://wholehousefanguy.com/
Here are links to the popular sources for rafter mounted fans
Here are other suppliers. These make the old fashioned ceiling mounted fans.
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Kurt Shafer YOUR Whole House Fan Guy 800 579 5018