The phrase "most ad hoc" can be replaced with several synonyms based on context. One synonym is "spontaneous", which means something that happens on the spot without any preparation. Another similar word is "impromptu", which means something created without forethought or planning. Additionally, "makeshift" can be used to describe something that is put together quickly with whatever materials are available. "Stopgap" can also be used when referring to a temporary solution to a problem. "Patchwork" is another synonym that suggests something assembled from various sources or pieces. Finally, "haphazard" describes something that is disorganized or lacking a clear plan.