An element meaning “sky” in several names from The Etymologies of the 1930s: N. Elfaron “Sky-hunter” (Ety/SPAR) and N. Elthoron “Eagle of the Sky” (Ety/THOR). It was derived from the root ᴹ√ƷEL “sky” which had an Old Noldorin form: ON. elle (Ety/ƷEL). However, Tolkien said “In Noldorin and Telerin this is confused with EL star”, implying that the word was not used in modern language; an earlier but rejected version of this entry had archaic N. †ell, el “sky” (EtyAC/ƷEL).
Neo-Sindarin: Despite the above statements, ell is probably the best attested option for “sky” in Neo-Sindarin, and I would use it as such, since it is in fact distinct from S. êl “star”, a word that is itself archaic/poetic versus more common S. gil.
A word in The Etymologies of the 1930s glossed “hole, gap” derived from primitive ᴹ✶gassā under the root ᴹ√GAS “yawn, gape” (Ety/GAS). Based on context, I think this word refers mainly to a hole or gap between things rather than a hole inside of something.