My son went to remove the fuel tank access cover (he's done this successfully before) with the Volvo tool I have and because it is so rusty pretty much knocked out the nibs. Nubs? Nibs? Now there's not much left to use to try to turn. Any ideas how I can safely try to remove this without blowing myself up and/or getting rust into the tank? Thanks.

The bung-removal tool almost never works for me but a flat screwdriver hammered usually does.
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240s: 2 drivers and some parts cars

you will find when the locking ring is hopelessly rusted away the bulk of the ring itself is sufficiently weak to allow the use of a flat blade screwdriver inserted between the ring and the tank sealing flange in the few areas of the ring which are NOT locked under the flange.

once this is done use leverage to bend inward the locking ring. it will deform and lose all integrity very quickly. then use a pliers to easily remove the remains.

from start to finish 1 minute or less