A word appearing as ᴱQ. etl “ivy” in the Qenya Lexicon and Poetic and Mythological Words of Eldarissa of the 1910s derived from the early root ᴱ√ETE “cling together” (QL/36). In the Qenya Lexicon it had an alternate form etil.
Neo-Quenya: In Tolkien’s later writing, √ET meant “out(side)”, but I think this ivy word can still be based on this root in the sense of “✱a thing clinging to the outside”; I would thus use ᴺQ. etil “ivy” for purposes of Neo-Quenya.
A noun appearing as G. ethl(in) “ivy” in the Gnomish Lexicon of the 1910s derived from primitive ᴱ✶etḷ-, where tl became thl in Gnomish of the 1910s (GL/33).
Neo-Sindarin: There is no sign of tl > thl in later Sindarin, so I would adapt this word as ᴺS. edol “ivy” for purposes of Neo-Sindarin.