This word had a quite lengthy history as an element in the name S. Glorfindel “Golden Hair”. It appeared in the Name-list to the Fall of Gondolin as G. findel “tress” (PE15/24) and in the contemporaneous Gnomish Lexicon with the gloss “a lock of hair”, but in that document it was deleted and replaced by G. finn “a lock of hair” and G. fingl or finnil “tress” (GL/35). It appeared as N. finnel “(braided) hair” in The Etymologies of the 1930s under the root ᴹ√SPIN (Ety/SPIN).
In Notes on Names (NN) from 1957 findel was an adjective meaning “having beautiful hair” or “having fine hair” (PE17/119, 151). In a torn half sheet from the late 1950s or early 1960s it was OS. findel, S. finnel “mass of long hair”, and in a document from around 1965 it was (archaic) findel “head of hair, fax” from primitive ✶spindilā (PE17/17). In this last document Tolkien said it was “preserved mainly in such old names as Glorfindel”, so Tolkien may have intended that it was no longer in active use in modern Sindarin.
Neo-Sindarin: For purposes of Neo-Sindarin, I’d use fîn for a single hair, find for a tress or lock of hair or hair in general, with finnel used for an entire head of hair, assuming finnel survived into modern Sindarin with the usual sound change of medial nd to nn. I would assume it can also be used adjectivally in reference to having beautiful hair.
n. single hair (of man or elf). >> find