The word "well-set" refers to something that is firmly established, sturdy, or physically fit. Antonyms for well-set could be flimsy, frail, weak, unsteady, or unstable. Alternatively, antonyms could focus on the opposite meanings of "well" and "set." Words that could be antonyms for "well" include poorly, badly, or insufficiently, while antonyms for "set" could include unset, movable, or incomplete. Ultimately, the choice of antonym will depend on the context in which the word is being used and what specific aspect of "well-set" the writer is seeking to contrast.