When expressing the idea that something is situated atop or following another thing, there are various phrases that can serve as synonyms for "on back of." One option is "on top of," which straightforwardly indicates that something is positioned above something else. Another option is "following," which can convey the idea that one thing is trailing behind another thing. "Alongside" or "beside" can be used when two things are situated next to each other rather than one on top of the other. Additionally, "upon" or "atop" can serve as more formal-sounding equivalents of "on back of." Ultimately, the choice of synonym depends on the context and intended meaning of the sentence.