Well, it seems that handwaving is in the eye of the beholder. The issue is mapping Egyptian cosmogony (or strictly, that of Heliopolis - even Egyptian mythology was not consistent) to the physical world - on the big assumption that the Egyptians saw the world in physical terms, rather than divine.
My not mentioning the stars in my article is scarcely relevant unless one determines what stars are - amongst the various Egyptian stories they represent the souls of Nut’s children. Divine beings, again… but it was believed that the stars also inhabited the afterlife in the underworld - the five-pointed stars on Nut were used to represent the afterlife, Duat - which is the very reason why most of such representations are from coffins. The picture is more about life after death than physics: getting it to correspond to some purely physical model is tricky.
If we start from the visual representation itself , the conclusion that the sky goddess, Nut, is necessarily representing a solid vault cannot be made. For we see in the picture she is supported by her father Shu, the air (understood primarily in spatial, not material, terms - and certainly not solid). Indeed Nut’s mother was Tefnut, moisture. And her brother, above whom she was lifted primarily to stop them having constant incestuous intercourse and lots of children, is Geb, the earth. Clearly the gods’ “constituents” reflect their roles. Nut is whatever the sky is - except that primarily, to the Egyptians, she is divinity.
Ra the sun god in some versions sails up her legs and over her back - but in others, enters her body each day and emerges from the other end. Nut is often represented graphically by the ladder by which her son Osiris escaped to safety within her body - which therefore appears to be porous, at least. To say she’s the direct equivalemnt as a supposed metallic solid vault “like a mirror” is stretching analogies, at least.
The main imagery of Nut in the mythology is sexual, an image of desire and fertility in relation to the earth: it’s more important that she’s separated from the earth than that she’s holding up anything else. Now, it’s true that one (small) element of Shu’s holding her up is that, should earth and sky meet again, chaos would return. But it’s another thing simply to resolve the complex mythology of Nut into a physical barrier against physical water - and then say it’s directly equivalent to the Hebrew raqia, whatever that may be, or Enuma Elish’s goddess-skin. All they have in common is representing “sky”. The Heliopolis myth itself shows that.
In the Heliopolis cosmogony, the primaeval situation was Nu, the endless waters. One should remember, though, that this concept is believed to have arisen out of Egypt’s annual inundation by the Nile, and the experience of land re-emerging afterwards. For this primaeval water (like that of Genesis 1) has a surface, from which the primaeval mound, and/or Ra emerging from a floating blue lotus, appeared. As I have written regarding Genesis, our modern interpretation of “boundless” as extending infinitely in all three dimensions matches neither the ancient descriptions, nor the concepts available to the ancients. This is not handwaving, but addressing the text as it is - if a surface is mentioned, as it is both in Genesis and Egypt - and in Enuma Elish too, actually - the “water in all directions” scenario is in trouble, and therefore the idea that it was thought that there was an enless mass of water above the sky to keep out…
At this point, visualising the theogony in terms of our “ANE/Genesis standard” picture gets difficult (and easily involves handwaving rationalisation to modern worldviews), because Ra, doing his various procreative acts, is operating in the world above the surface of Nu, the waters, not in some bubble within it.
Likewise, the waters themselves embody a richer concept of “chaos” than simply that of physical inundation by a massive body of water above the sky. For chaos is contrasted with order, Ma’at, which all Egyption religion sought to preserve and which was not primarily a material concept, but “the ancient Egyptian concept of truth, balance, order, harmony, law, morality, and justice.”
Now I’ve cited all this stuff, that’s pretty alien to our view of the world but was the Egyptian view of the world, to show once again that it is not “handwaving” to urge caution in trying to reduce disparate ANE cosmologies either to some generic picture, nor yet to some supposed “ancient science” comparable to “modern science”.
On another point, it’s also irrelevant to use 2nd temple Jewish and early Christian authors to interpret Genesis cosmology. In the first place, we’re apt to see even those sources in modern materialistic terms. More relevantly, they all all date from millennia after the ANE source, and were all influenced by the Hellenistic worldview broadly, if anachronistically, describable as “Ptolemaic”.
But since I see we are coming from completely different directions, I simply leave this to other readers to take into consideration. But I’d be happy to comment on some of the many other visual representations of non-Heliopolitan cosmologies that you mentioned way back in this thread (and whose existence others seem to take somewhat for granted).