The /ʊə/ sound is spelled with <u> or <ur>.
As /ʊə/ is fairly rare, only occurring in 1053 words in the dictionary, this was a fairly easy list to work out manually. Since the diphthong is often preceded by a palatal semivowel /j/, I have treated this as an optional component of the diphthong. For example, in the first five pairs below, I have actually deleted /jʊə/ rather than /ʊə/, but I left these pairs in as they seemed interesting.
The density value and semantic loading are irrelevant with such a small and contentious list.
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John Higgins, Shaftesbury, August 2012.