What is being used for the moist but not wet substrate in the box? Pehaps whatever it is, along with the moisture, is holding the heat to a greater extent than the rest of the enclosure, having less ability to vent, like a compost pile in a storage shed. The box might well be absorbing heat. A dark box might heat up quite a bit in proximity to lighting. Sure, rather obvious, but the simple is often overlooked.
Guy