A rejected root in The Etymologies of the 1930s with a strengthened form ✱ndē̆ serving as the basis for the prefix in N. nestag- “insert”, possibly related to ᴹ√NED “middle” (EtyAC/NĒ̆). The strengthened form may have been replaced by the root ᴹ√(N)DI “in” (EtyAC/NDI), but in later writing ✶ndī was given the meaning “beneath, not touching, under” (PE17/95). Despite the rejection of the root, the verb N. nestag- “insert” remained unmodified under the root ᴹ√STAK (Ety/STAK), and there is further evidence for the ne- prefix in the verb N. neledh- “enter” appearing on the 1936 draft of Thror’s Map from this same time period (TAI/150).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is worth retaining this (Noldorin and Sindarin-only?) prefix ne- “in”.