Hi Sam, an icebox is not designed to keep food warm. It is designed to keep ice frozen for as long as possible.
For warming, you could consider a Thermoelectric Cooler/Warmer product; many of these models have the capability to cool down to below around 20- 25 °C or heat to 55- 65 °C above the outside temperature.
These units will require electical power to run (either 240v or 12/24v).
Many thanks.