Wheel locking nuts are a great accessory for mounted spare wheels. They have a unique drive head on the top of the wheel nuts which prevents just a standard 19mm socket drive being used. To use the wheel locking nuts it requires the key piece (2nd Photo) and requires a 21mm socket drive.
$37 (Incl GST)
Mounted Spare Wheel with Locking Nuts 2.
Wheel Lock Nuts Main
Wheel Lock Piece