Drill four 3 mm holes into the wood and glue the four rod magnets into the holes.
Wrap the magnets well with copper wire.
Glue the two plugs on the wood.
Solder the two pieces of thicker copper wire on the top of the plugs.
Glue the wood under the mandolin strings.
Bring the thicker copper wires together and connect them to the amplifier (in this case via a socket).