This is my newest unit, running a 510 atomizer with a Texas Instruments 5v regulator. It won't get hot enough to damage the box, but the batteries do get slightly warm with frequent use from being in the enclosed space w/ the regulator so I try not to chong on it nonstop. The regulators are so nice and small, you can fit them in almost anywhere.
Major big thanks to Shreck for the links and advice with the regulator on this one.