I'm working on mine now. I am using worbla with thin eva foam floor matting for support. nylon straps and plastic buckles to keep it on. When I get it more done and I'll help. Not sure if youre still working on this. Just trying to help.
Here's a link to Adam Savage's build from when he made a Doc Ock for Patton Oswalt. It's a parent-friendly quick version using lightweight foam for the arms, so that may need some reinforcement if you're going heavier. Shots of the harness at about 1:38 or so.