Peter, Have mentioned this many times so ignore if you are already going along this path.
When you pick up your detector with bungee cord only does it hang in detecting position? ie: The coil is hovering just above ground level?
With bungee connected to your harness, preferably using the spreader bar to distribute weight to both shoulders, does the detector remain in detecting position if you remove your arm?
Have seen many set ups where the bungee is connected to the stem either just above coil or just below box, this will definitely cause fatigue and discomfort in a very short time.
Definitely NOT a young fella here but 4hrs of swinging the Z between breaks with the 14" coil is a breeze.
Good luck mate.