The panel with the 'U' shaped hole has a duplicate one on the bottom of the cube. The problem is, the shoulder would just fall off if I tried to put it on like that. I made a 'box' which is actually more true to how Optimus Prime looks (the lower half of his upper arm). The bonus is that the box helps hold the thing in place.
I'm thinking that since I'll have to wear a red turtleneck underneath (you'll *never* catch me wearing a turtleneck, ever... except for this. I promise.) and I could use a bit of velcro on the shoulder to hold it up there. That's about it, it's a rather uninteresting piece; no crazy engineering here.