This Tower shelf, by Japanese manufacturer Yamazaki, provides a little more storage space in the bathroom. The no-installation design simply piggybacks on the towel bar:
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I do think the hooks in front are a bit much, as they’d interfere with removing/replacing the towel.
Without the hooks, if this were sized up to U.S. towel bar proportions, I could see this doing well here (maybe not in the MoMA store, but on the QVC network).
Source: core77