The phrase "made up off the top of one's head" means to improvise or come up with something on the spot without preparation or careful thought. Antonyms for this phrase could be "well-planned," "meticulously researched," or "carefully crafted." These terms refer to something that has been thought out, researched, or prepared in advance, rather than being created spontaneously. Other antonyms could include "factual," "accurate," or "verifiable," as these words suggest that the information or idea presented has been carefully checked for accuracy and can be backed up with evidence. Overall, the antonyms for "made up off the top of one's head" convey a sense of professionalism, accuracy, and careful attention to detail.