N. tunn “hill, mound” appeared in The Etymologies of the 1930s as a derivative of the root ᴹ√TUN (Ety/TUN). Tolkien’s continued use of Q. Túna for the name of a hill in Valinor implies the ongoing validity of its root, so perhaps this word remained valid as well.
Conceptual Development: The word ᴱN. tûn “mound, bare hill” appeared in Early Noldorin Word-lists of the 1920s (PE13/154). G. tûn also appeared in the Gnomish Lexicon of the 1910s, but there it was unglossed and had no obvious cognates, so it is not clear what Tolkien intended it to mean (GL/72).
Neo-Sindarin: In keeping with the rule whereby nd was retained “at the end of fully accented monosyllables” (LotR/1115), I’d represent this form as ᴺS. tund in Neo-Sindarin.