Were it me, I'd try and match the finish of the adapter to that of the Shimano casting. A buff with some Solvol Autosol.
Applied using a Dremel with a polishing mop, it wouldn't take too long.
A coat of clear varnish and Robert is your Mother's brother.
ETA - If you haven't got a Dremel, stick the adapter in the post to me and I'll do it for you. I think you have my postal addy.
ETFA - I'd also radius all the square edges of the adapter slightly, to soften them and bring it aesthetically closer to the Shimano casting.