The phrase "knocked together" generally means hastily or crudely assembled. Therefore, the antonyms for this phrase would be more refined and carefully put together. Some possible antonyms include "crafted," "built," "constructed," "fashioned," "fabricated," and "assembled with care." These terms denote a sense of quality workmanship, attention to detail, and a thoughtful approach to creating something. Using these antonyms, you can highlight the difference between something that is hastily or carelessly done and something that is carefully crafted with skill and expertise.