No offence shooter your harness looks well made and i'm sure it works but
A few things to take into consideration in the making of a bungee.
The closer the point of anchorage to the actual shoulder the less resistence it will give on the swing and give a complete and even arc with little or no restriction, as with your arm.
Having it low down will give resistence either one way or the other, in the picture swinging to the detectorists right will cause the strain to come on the chord, to the left not so much
A longer chord will also give a wider arc with less resistence although still supporting the weight, all you should need to do is guide it from one side to the other.
Having the point of anchorage at one side will also keep the detector at that side when you let go, if its in the middle that is where it will go when you let go and will get in the way when digging etc.
Clipping the harness to your trousers is a great idea in the dry but its no good when its raining as to clip it on you would need to raise your jacket and end up with a wet bum as its not covered
Please dont take offence, these are only suggestions to make a good thing better