Looks like you have about 10 cf (cubic feet) I'd use a temperature based controller to drive a fan that will pull in about 30 cf (3 x the box volume) of fresh air per minute. Even using CFLs, it can get warm so pulling cool air in is a help too, meaning the air outside of the box should be as cool as possible. Got any pictures of your grow box? Check the links in my signature to see what I was doing with a grow box.. I had a 600w HPS packed in mine.

Farmer Rich