How can I cool down a hot garage?
What about an attic vent fan such as this one?



It has a thermostatic control. 

Separate shutters are also available such as these.  They open based on the air pressure generated by the fan.



You would need to create "in" vents, preferably placed low on several walls to allow full air flow. These could be either fixed or shuttered, depending on whether you are concerned about infiltration of air in colder weather.