Take a piece of flat steel about a metre long, the width just slightly less than the diameter of the bung. Bend the last 5 cm or so at right angles to the main piece, and voila! you have a good leverage to open the bung without scoring or damaging the drum and/or bung.
Now, go to the other end, grind down the width to the size of the small air vent bung, and bend it 90 degrees in the other direction.
It worked well for my grandfather, who handed it down to my father, who handed it down to me, and my son.....buys gas from the bowser.