I'm not an expert on grow boxes (yet) but when I built my computer I designed an extremely efficient airflow pattern. Looking at you box I think you might want to try elevating it at least an inch off of the ground if it isn't already, then cutting holes very low in the sides or in the bottom corners, and a couple in the top above the lights. I think a couple of decent computer fans would be able to pull more than enough heat out if the exhaust is right above the lights, and you can use a $5 computer power supply pluged into your light timer. The only part that may be any trouble is building light baffles, maybe someone else has an idea for that? Let me know what you think.

l3lunts