that cigarette socket will be OK for a 42V 2A charger.
the 300/600W is the maximum output of the converter, it will only output what is drawn by the charger. If your charger is not a fast charger, you will be OK. Read the label on the charger, you'll see something like max output 42V 2A. Add 15% for conversion loss, you get to the maximum power draw of the charger, typically about 90W-95W. Well within the specs of your cigarette socket (360W on 24V).