I think the harness was mostly concealed by the jacket.
Consider leather guitar straps, adjustable, and some widen over stress points. The wider the strap over your shpulders, the more friction and the less it will want to roll.
I made a comic Rocketeer build and I put the rocket on a block of wood covered with fake leather. I then used leather straps to carry it like a backpack. You will have to play with it to get it to sit right. The rocket will want to roll down your back to its lowest point.
A 3D printed build I would try to have something inside the rocket to give it strength. Wood or aluminum,maybe poured or aculpted expoxy . You need atleast 2 points of contact from rocket to harness, four is more stable.
It will bump into things, doorways, walls, you dont want it to accidentally crack it. Dont just sand and paint.
Also, some safe way to put it down when you need a break.
Since you printed in black, coat with black pigment in XTC 3D, then sand it down super smooth and spray it with a chrome spray pantt to get that metal, slightly reflective look.