The verb G. hel- “freeze” appeared in the Gnomish Lexicon of the 1910s as derivative of primitive χele-k (GL/48), based on the early root ᴱ√HELE as suggested by Christopher Tolkien (LT1A/Helkar; QL/39).
Neo-Sindarin: Since √KHEL appeared in later writings with glosses like “freeze” (Ety/KHEL) and “ice” (PE17/116), I think ᴺS. hel- “to freeze” can salvaged for Neo-Sindarin.
A transitive verb in the Gnomish Lexicon of the 1910s glossed “freeze, congeal (tr.)”, apparently a transitive variant of G. hel- “freeze” (GL/48).