March 27 2019 Thorwaldson is now offering the rain hat as a separate option for adding it to all other models of rafter mounted fans to enable them to be used with no attics.
You can now choose from various throat sizes to fit the system you are using. Here is a page from Invisco Industrial showing some of them http://inviscoindustrial.com/prices/
As an example, Airscape, Tamarack and VentCool have fans that come in 24 inch diameter shells. The Tornado (24 inch) gravity ventilator is perfect for their fans. Here is the top in final test. It is priced at $695.
PERFORMANCE IS GREATER BECAUSE STATIC PRESSURE IS NEARLY ZERO
This Tornado rooftop has wide open spaces between thin vertical turbine blades. As a result tests show a 30% increase in air flow. So if you use a 24 inch fan that pulls 5,200 CFM in an attic, it will pull over 8000 CFM through your roof.
By comparison, another source for these high performance rooftop ventilators/rain hat is Edmonds Australia. They have a United States Distributor, EdmondsUSA, that sells the Hurricane H600 (24 inch) for $965.