I agree with DVH, the back armor only needs to be attached to the collar, which is attached to the vest, with the back plate behind the bar. Then the lower hooks on the jetpack rest on this bar with the straps coming through the back plate & using the rings to hang the upper hooks on. The harness may push the back plate out slightly but it should sit between your shoulder blades & fit in the small of your back. If set up right it is very comfortable & doesn't pull on your shoulders.
Here are a couple of photos of how mine is set up & sits.
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