hands down a hps lamp will out perform a cfl any day of the week "not to offend the cfl growers, u do good work!!" the box in height is 2 small and your really going to be kicking your self later.5 to 6 ft is optimal "it also gives the heat a place to go to thats not right at the canopy.my suggestion,buy the hps lamp buy a decent cfm fan and some exhaust tubing.vent the light with the fan pulling the air through a filter and out of the house.drill several 4 inch holes at the bottom side panel of your box 4 or 5 it perfect.at this point u wil have fresh air from the room your box is is full of usable natural co2 and u have solved sent and light heat issues all with one fan.post some pics of your setup!!