There are several synonyms for the phrase "not further than" which can be used to convey the same idea of a limited distance or boundary. Phrases such as "within a certain distance," "limited to," "restricted to," "short of," and "close to" can be used in place of "not further than." These phrases indicate a specific range or boundary that cannot be exceeded or surpassed. It is important to use the appropriate synonym in context, as the intended meaning of the sentence may differ slightly depending on the choice of phrasing. Ultimately, all of these synonyms emphasize the idea of a measurable or contained distance.