to get you off his forum, ima just keep following you around till you stop being wrong diaz. if your ehaxust vent is at the highest point on your box you don't need more then a 12" computer fan, it's an exhaust fan durp, you can use a damn Chinese fan is you got creative enough, it's your intakes that need to be changed, cold air likes to stay low cold air does not like to wander in and out of areas, thus a fan is needed to PULL the cold air in, hot air will keep rising until it finds a way out or makes it to the well highest point so cutting a hole large enough to allow the same amount of air out as that's being fed into the box provides the exact same results, if your investing into large exhaust fans or higher powered ones your doing something wrong.
he is right about the light leaks though... surprisingly, being a roofer i'm guessing you have an abundance or caulking and tar kicking around. duck tape your corners/ edges thin layer of tar then caulk over top of it all and it will not only be light proof but water proof too.