The phrase "make top of head" generally means to come up with something off the top of one's head, without much thought or planning. Synonyms for this phrase could include "spontaneously," "impulsively," "offhand," "unrehearsed," or "instinctively." These words all suggest a lack of preparation or forethought, and emphasize the notion of something being generated quickly or without much effort. Other possible synonyms might include "improvised," "ad-libbed," "off-the-cuff," or "on a whim." All of these terms convey the sense of something being done without much forethought or preparation, and can be used interchangeably with the phrase "make top of head".