Returning to this thread: in a little-noticed detail, Ralph McQuarrie’s 1975 designs for stormtrooper armor differed from the film version by giving lower-ranking troopers helmets with chrome tops.
This was probably the inspiration for Captain Phasma’s chrome armor in TFA (albeit used to signal higher rank rather than the opposite).
Also! Not a costume per se, but here’s a Ralph McQuarrie design for the Death Star torture robot, along with a probably-unrelated helmet design:
