Step 3: Epoxy the Plug
Now that you've got the plug all washered up, it's time to bring on the magnets. Roughen the magnets and washers with sandpaper to ensure a good bond, then mix up a batch of the strongest epoxy you're comfortable with (I'm personally uncomfortable with five-minute epoxies because they set up too fast, and I like to use an epoxy that I can cut off in case I mess up).
Make sure that your magnets firmly contact both the washer and the copper foil. The magnets will be your new ground contact, so make sure that they have a good electrical connection to the ground cylinder on the original plug!
Use aluminum foil and rubber bands to make a seal so that the epoxy doesn't run off the edges. Prop the plug upright, and carefully insert epoxy in the spaces between the magnets. You don't want to get any epoxy on the top faces of the magnets, and you don't want to get any in the hole in the center. But you do want to fill the spaces between the magnets with a little pool of epoxy, bringing it almost flush with their tops.
This will seem obvious in hindsight: don't use an iron implement like a nail to apply the epoxy.