i dont use an arnes to atach my jet pack
i just hook the JP to the backplate on the holes and the JP its very secure there, but after a few hours the problem start, why, because remember you are wearing a suit without much ventilation, so you start to get tired, i think the problem is common, but if you can make your JP more light start to trim parts or replace anothers the heviest 
check this pics, i take this in the wip status of my actual suit 
...and sorry for the bad quality 
<img src=http://200.72.134.171/cdref/tdh/img/jp10.jpg>
<img src=http://200.72.134.171/cdref/tdh/img/jp11.jpg>