You will run into all kinds of heat problems. The unit will overheat and die if it's not got ventilation through the hood. You'd need to install a couple of fans, one blowing in, one blowing out would get good air flow.
Even then, in summer you'll struggle to keep the tank temperature in check. Normally T5 units are hung above the tank, 8 to 12 inches above to give plenty of air flow and to avoid overheating the water.
LED units will heat the water less.