A natural convection cooled electronic device utilizing a box-like plastic enclosure surrounding the circuitry of the electronic device. The apparatus further includes aluminum heat sinks fastened in good thermal contact with heat dissipating components of the electronic device, wherein the heat sinks include heat fin members which run parallel to the inner walls of the enclosure and are separated from the walls by an air gap. The inner walls of the enclosure are lined with a layer of thermally conductive material, such as copper foil, which spreads the internal heat flux across the total surface area of the enclosure. The exposed surface of the heat flux spreader layer and the facing surface of the heat fin members are further made semi-rough and stained black to eliminate potential hot spots and to increase radiant heat transfer between the heat sinks and the enclosure. An alternative embodiment includes an electrical insulation layer between the electronic circuitry and the heat flux spreader layer. The insulation layer may include openings to allow contact of the heat flux spreader layer to the electronic circuitry at grounded locations.