You can PC stainless but from what I read, it can be tricky. Normally metal is shot blasted to make the PC adhere. Personally I don't think that there is any advantage in stainless for a speaker bracket, even if you are on the end of a pier? Stainless is harder to weld, meaning that you'd need to go back to the builder if something fails? Ok so you might anyway. Balance wise, a mate had a 4x10 trace Elliot cabinet that was a bloody nightmare to carry as it had handles centred on the box, and tipped forwards when carried. I built a FoH rack and calculated the CoG for handle placement so it was easy to carry.