well how bout getting a thermostatic control for the fan so you could set the exact temp you require. these are available quite cheaply but are generally for 3 speed fans. also if you duct your exhaust through the top of the cab, build a small box around the exhaust, (on top ofthe cab) then secure a mesh screen about an inch away from the bottom of the box, cover the mesh with some nylon stockings and then just pour some active carbon (available at any petstore) into the box, once you've got about 3 inches of carbon over the mesh voila effective odour control