I agree with Matt here. If over the heat is the only place for him to hide, then that is where he'll be. While the temps sound fine, it is still possible that overheating can happen. But if outside that box looks to open and scarey he won't come out. Is your heat mat on a thermostat?
I lost a hatchling python once after he was burnt from heat off a heatcord. It was on a thermo but it had stopped working. I failed to check my equipment to make sure it was all working properly. The only place I had for him to hide was over the heat. From dry belly to red raw burn in one day, I then checked my temps with my Infrared thermometer and knew straight away the thermo. had stopped working. When I thought he was responding well to antibiotics and burns cream, he bled to death whilst trying to shed, 4 weeks later.
Another thing to consider too is that diamonds are nocturnal. So its their natural right to hide during the day and only venture out at night. Try dimming the lights at night and see if he will come out. But I still recommend moving the hidebox off the heat. Near is fine, but not directly on it. I also recommend, when buying heat equipment, buy only as powerful as you need. Don't depend on a thermostat to keep the temps down, as one day it will fail, the heat climbs and will cook your pet. If you need a thermostat to stop the temps going too high, you need to downsize your heat mat.
