Major Charles Emerson Winchester III (played by actor David Ogden Stiers) had a "Boston Brahmin" accent and Stiers had it down pretty well. Even the OED says that a brahmin is "a highly cultured or aloof person especially a member of the upper classes of Boston, Massachusetts, USA."
What most consider a Boston accent is softer. For instance a Bostonian says gen'lemen where a Brahmin says gentleMEN a la Major Winchester.
I have never heard a regular Boston accent imitated well by any actor who was not born into it.
I cringe every time Cliff the Mailman says Bahstun on "Cheers."

No help at all wot?