heat dissipation in watts of equipment in a box
Your problem here is that you're looking at the wrong part of the thermal equation. The heat dissipation of a heated metal box is dominated by the thermal resistance of the metal/air interface, not by the thermal conductivity of the box itself. Characterizing the thermal resistance of that interface will be difficult without taking a lot more measurements.

From a practical perspective, the simplest solution will be to overdimension the heater and use a thermostat to keep the enclosure at the target temperature -- this will make the exact size of the heater unimportant, and will also mean that the temperature will remain stable even when the ambient temperature changes.