For my jet pack harness, I'm currently using a variation/hybrid of Boba makers and Fettdads systems. It's very simple in design, easy to put on and take off, dosen't slip or shift while being worn and easily holds the weight of the jet pack making it feel like it's not even strapped to your back at all.
The absolutely best thing about this particular harness is that it was relatively cheap to make ( cost me under $20 Canadian for all the materials ). I'll see if I can't post up a pic of the harness for you tonight.