The adapter is quite easy to dismantle. The case is usually a two part solvent welded moulding with the join about a third of the way out from the pins.
Put the top part in the vice so you have the plug pins uppermost and the long side in the vice jaws. Position it such that the weld line is just out of the vice jaws. Close the vice until the weld cracks (the case will deform but not break). Remove the adapter, turn through 90 deg so the short side is in the vice and repeat. The top will now come off the adapter quite easily and you can gain access to the back of the pins to fit a home made one properly.
Fix it all back together again with plastic glue or solvent weld (from your local plumbers supplies place). Test the firmness of the bond before giving it back to the anklesnappers.