If you are searching for synonyms for the phrase "made up off top ones head," you may want to consider the words improvised, extemporaneous, impromptu, unrehearsed, or spontaneous. These are all terms that describe the act of creating something without any prior planning or preparation. Other possible options could include ad-libbed, off-the-cuff, spur-of-the-moment, or on-the-fly. Ultimately, the most appropriate synonym will depend on the context in which you are using the phrase, as well as the tone and style that you are hoping to convey.