Search Home Depot website or whoever for 'Attic Fan' There are ones made just for this with brackets to mount to the wall studs around your vents. Uber reliable because they are made to be put up in hot inaccessible areas.
I've checked those out, but you can get a 20" fan with 3x more CFM for 3x less money if you don't go with the specialized attic exhaust fans. Plus, these fans are often used in warm environments like warehouses so I'm sure they would be able to handle it.