I have no experience trooping with a Fett Jet Pack, but I do have a Ghostbusters proton pack that weighs over 20 pounds. The problem is that your shoulder straps are too tight and you are using them to carry your pack. The shoulder straps are not meant to be weight bearing. They should only stabilize your pack to keep it upright.
All of the weight is meant to be borne by the belt. If your shoulder straps are tight and your belt is loose, you are doing it wrong. You will need to either tighten your belt or move it to a place that the weight is sitting on your hips (probably up). The straps should only be tight enough to keep the pack vertical. After several hours, your back should feel sore, not your shoulders. Especially with that harness, which is well designed to distribute the weight properly.