The sky being a vault, or dome or touchable FIRMament, like "terrible crystal" (Ezekiel 1:22), separating the waters from the waters.

I think it relates to the common Sky on Earth and Sky Earth Separation theme in myths from on around the world. Often people/ancestors/heroes/gods seem to have lived in a hollow between Sky Father and Earth Mother. Later in time both always get separated.

Now for the interpretation - What is Sky Father?

Is it the sky as we know it? We can't touch it, so probably not. Suppose it actually is something touchable, that looks like "terrible crystal", maybe even looks like the sky that we know. Something where people could take shelter in like in a vault, or cave.

I think the Sky Father is some kind of (ice age?) glacier and the hollow between Sky and Earth is a glacial cave. Ice is a terrible cold crystal, firm, touchable, looks like the sky. Ice caves exist, but they dont provide a great home, but they separate the waters above from the waters below. There is always something to find with subterranean water in mythology.. Now if there is a geothermal spring below a glacier or chunk of ice, it could form a survivable space, that ticks all the marks.

Many myths from all around the world share those elements to some degree, but I will here point only at one particular example, from - least expected - the Māori, that I find especially interesting, because of its mentioning of "extreme cold".

https://www.sacred-texts.com/pac/lww/lww3.htm

Some excerpts (my comments marked with > at line begin):

The Rangi-nui [great sky], which stands above, felt a desire towards Papa-tua-nuku [the earth], whose belly was turned up [towards him]; he desired her as a wife. So Rangi came down to Papa. In that period the amount of light was nil; absolute and complete darkness [po-kutikuti kakarauri] prevailed; there was no sun, no moon, no stars, no clouds, no light, no mist—no ripples stirred the surface of ocean; no breath of air, a complete and absolute stillness.

And so Rangi-nui dwelt with Papa-tua-nuku as his wife; and then he set [hikaia = whakato, to set, plant] plants to cover the nakedness of Papa; for her armpits, her head, and the body; and after that the smaller trees to clothe them both, for the body of the earth was naked. Subsequently he placed the upstanding trees of the forest, and now Papa felt a great warmth, which was all-embracing.

> and some more.

Because Rangi-nui overlaid and completely covered Papa-tua-nuku, the growth of all things could not mature, nor could anything bear fruit [or increase]; they were in an unstable condition, floating about the Ao-pouri [the world of darkness], and this was their appearance: some were crawling [after the manner of lizards], some were upright with the arms held up, some were lying with the knees partly drawn p. 118 up, some lying on their sides, some were lying stretched out at full length, some on their backs, some were stooping, some with their heads bent down, some with their legs drawn up, some embracing, some kicking out with legs and arms, some kneeling, some standing, some inhaling deep breaths, some with exhausted breath, some crawling, some walking, some feeling about in the dark, some arising, some gazing, some sitting still, and in many other attitudes—they were all within the embrace of Rangi-nui and Papa.6

> Sounds cramped and not a nice place to be..

It was after this manner that they dwelt in Pō, within the space included in the embrace of their parents. It was very long that condition of affairs existed; until at last a faint glimmering of light, a scintillation like the light of a star was seen, or like the Will-o-the-wisp at night. And now commenced a desire on the part of the family of gods to go forth from between their parents to follow the faint appearance of light. Some of them [the gods] consented, some did not; and thus it became a matter of strife between them. Tāne (68), Tu-pai (69)11 and others said, "Let us seek a means by which we may go forth." The matter was then assented to.

Now, at a certain time after [that discussion] Ue-poto (28) went to bathe and wash away the clammy feeling arising through the warmth of their dwelling place [within the embrace of their parents]. He was carried away outside on the current of their mother's urine, and found himself outside in a gentle cooling breeze, which was sweet scented in the nostrils of Ue-poto. He thought, this is the best place, here outside, so he called out under the sides of his parents: "O sirs! Come outside, for this is a pleasant place for us."

> urine could be hint for geothermal activity and accompanying sulfuric smells

When the menstruous time of their mother Earth came, then Tāne (68) came forth. This was in the seventh Pō, or age of their desire to search for the "way of the female"12 in order to go forth. On reaching the outside world, they then saw that it was indeed a pleasant place for them [to dwell]. There was, however, a drawback; for the different kinds of the cold of Heaven [or space], Wero-i-te-ninihi, Wero-i-te-wawana, Kŭna-wiri, Maeke, and Kotokoto, there spread out their intense cold.13 Hence, did Rangi and Papa closely embrace [to exclude the cold from ther offspring], and, hence, also originated the 'goose-flesh,' and trembling through cold [from which mankind suffers to this day]. These are the enemies that afflicted the family14 of their father. In consequence of this, they sheltered under the sides of their mother, where they found warmth, which they named Whakaruru-taha [shelter-by-the-side], which name came down to us, and is applied to a warm and pleasant place where no winds blow.

> Here we learn about the cold outside. Later on Tāne climbs up to the Sky and deals with some knowledge preservers

Now, when Rangi-nui had been properly placed in position as is now to be seen, the blood from the arms dripped down on to Papa [the Earth], and hence is the horu [red oxide of iron], and the pukepoto [blue phosphate of iron], that his descendants in this world use in painting.19 And hence also is the red appearance that inflames the skies at sunrise or sunset—that is the blood of Rangi's arms.

> This sounds a lot like the Blood Falls (https://en.wikipedia.org/wiki/Blood_Falls) an outflow of an (red) iron oxide-tainted plume of saltwater, flowing from the tongue of Taylor Glacier onto the ice-covered surface of West Lake Bonney in the Taylor Valley of the McMurdo Dry Valleys in Victoria Land, East Antarctica. The part of Antarctica that is closest to New Zealand and the Māori. As it is the blood of Rangi's [sky's] arms, Rangi is actually the Ice of Antarctica, or more generally glacial ice.