The phrase "making up off top of one's head" refers to creating something without prior planning or deep thought. Its antonyms include carefully considered, thoroughly researched, and deliberately crafted. When one is not making something up, they are being methodical and meticulous about their approach. They are also taking their time to ensure that the final product is well-thought-out and polished. Some other antonyms of making up off top of one's head can be well-planned, methodical, calculated, systematic, and prepared. By using these antonyms, one can convey a sense of professionalism and seriousness towards the task at hand.