A quality pack like a Camelbak works very well, but it must be adjusted properly.
Have it loose fitting and the weight of the detector will make is very uncomfortable.
Prior to attaching the bungy, just tighten all the straps to make it part of your torso, and you won't know it is there.
One thing we added was some lambswool seatbelt overs to the straps where they come over the shoulder.
Once set up you can permanently attach you audio booster, speaker etc, so it is an easy process to get going.
I have a Camelbak Mule. A tad dearer than a harness but worth every cent, and can house all of you gear plus water.