Add extra insulation to heated lunch box

I have a Travelisimo electric lunchbox that I plug into my pickup truck's 12v cigarette lighter socket. 10-in x 7.5-in x 4.5-in.

The device works well it but takes 1-2 hours to heat up a small amount of frozen food due to the limited wattage available in vehicles.

I want to speed it up by adding an extra layer of insulation that is easy to remove to access the food. What are my options?

A polystyrene box is the easiest thing that comes to mind. Just choose one with the appropriate dimensions. Worst case, buy some polystyrene sheets / boards and glue them together to the needed dimensions. You will probably need to carve a small slit for the electric cable.