Hodgies in Amesbury, MA is along 495. They are best known for their ice cream, but they have (or had, it's been a couple years) a decent, reasonably priced lobster roll.
Truth be told, I've never found a great one on the route you describe. In Maine there are many places, North of Boston, near the coast (Ipswich down to Winthrop) there are quite a few as well.