A word for “mine, boring, tunnel, underground dwel[ling]” in Notes on Names (NN) from 1957 with Sindarin cognate S. fela, both derived from ✶phelgā (PE17/118). In the same note Tolkien seemed to consider replacing it with felco “cave, mine, underground dwelling” from the root √PHELEK.
Conceptual Development: A similar word ᴹQ. felya “cave” appeared in The Etymologies of the 1930s derived from the root ᴹ√PHELEG, with cognate N. fela (Ety/PHÉLEG). The latter form also appeared in notes from 1969 as S. fela “minor excavations, den”, again derived from ✶phelga indicating √PHELEK was a transient idea (NM/304).
Neo-Quenya: I prefer the root form √PHELEG over √PHELEK and thus would retain felya, but I would keep its 1957 meaning “mine, boring, tunnel” rather than adopting the 1969 Sindarin sense “den”.