Icecreamslick, having it completely closed and using outside heat-sinks for it will not work, it should be ventilated, with some small openings with net-protection, if you use any plastic gears it should be ventilated.
You are right icecreamslick If using all metal-gears then go with closed gear-protector and heat-fins on the outside).
Or just go for the main source of heat problem, put some big heat-fins on the clutch bell
(edit: i meant clutch bell, for those who didn't already understood that by me just typing clutch
)