While nowhere does the Bible explicitly state that the 'raqia' is solid or firm, there was a longstanding and well established belief regarding cosmology in antiquity which implies it. Furthremore, the the firm nature of the firmament is inherent in the word רָקִ֖יעַ itself. The Hebrew and Aramaic Lexicon of the Old Testament shows that the word רָקִ֖יעַ (raqia; H7549) derives from the Hebrew word רָקִיעַ, the spread out, stamp out or down (as with a foot) or hammer out. This itself derives from the Akkadian word for metal bowl or kettle.
The sky dome would then have been understood as being firm based on this word alone. To interpret the firmament otherwise would require a complete rejection of the Lexicon - including older less reliable Lexicons like the BDB.
Acceptance of the idea that The Bible depicts a round earth which orbits the sun with a soft atmosphere requires us to believe that the Bible authors had a secret knowledge about the universe which they referenced no where else and that they described using identical language and vocabulary to other creation stories in the Levant but intended it to be interpreted differently than those other authors.
The Genesis creation and flood stories appear to be a polemic corrective to the other creation and flood stories in the Ancient Middle East. In that vein, should Biblical authors have had a different cosmology, we would expect to see that hammered home (pun intended) in the polemic. Instead we see the same vocabulary of the surrounding cultures and no discussion about things like heliocentricism and creation ex nihilo.
The other major point at which we would expect to see this discussion is in the talmudic wrestlings with Plato's work. We simply don't see that.
While I may not have a fancy diagram to show, I do have this depiction of the world from the Babylonian Imago Mundi (c. 500 BC) which is the oldest map of the world known to exist
This map depicts a pangea-like disc shaped continent floating in an ocean. Were one to depict a benben or the primordial primordial mound seen in nearly all the creations stories, it would most certainly look something like this.
A map with translations of the labeled points of interest is as follows:
I am unable to find a translation of the inscription at the top, but did find that it indicates the map is a copy of an even older map. This is significant because the final redactor of genesis (under JEPD theory) is typically dated to the same era as this map (c. 500BC) yet the writers of Genesis 1 and the redactor did not see fit to correct the Babylonian misunderstanding of cosmology - indicating they did not see a misunderstanding.
Similarly, Gensis 2 and 3 appear to be (primarialy) a polemic corrective to Egyptian creation myths. Tradition holds that Moses wrote it after the Exodus, the most likely date for which is some time during the 18th dynasty (BC 1549-1292). Therefore, the fact that we have a depiction of the firmament from Egyptian Creation myth on the Tomb of Seti I (1290–1279 BC) is extremely important.
From the Tomb of Seti I
Copy of above from the Book of Nut
From the Coffin of Priestess Amon Gautsesjen (C. 1070-712BC)
Geb (earth) and Nut (sky) being held up by Shu (air)
Copy of above from the Book of Nut
These depictions are from earlier written inscriptions dating back at least to the Pyramid of Unas in the 5th Dynasty (2360 BC) and probably earlier.
If the Hebrews had some secret knowledge about the correct cosmology of the universe, they did not see fit to discuss or correct the Egyptian viewpoint - even during their enslavement in Egypt.
Therefore in all likelihood it is because they did not view their world any differently from the cultures they lived in. They did, in fact believe that a fixed, solid dome existed above them just like all of the other cultures around them. While some like R. Laird Harris may believe that this is the result of scholars' ingenuity, it is Harris who is engineering an interpretation that simply isn't there - we don't have a single shred of evidence supporting Harris view. This is why the overwhelming majority of scholars link the Bible's view of the cosmos with other ancient cosmologies - because the evidence against this view is lacking (significantly more so than the alternative).
While an allegorical interpretation of this might be tempting, we simply don't see any evidence for that either. The ancient people came up with their cosmology through observational science - observing what things looked like and then guessing what they might be. The firmament cosmology is a logical conclusion of that limited way of looking at the world. Simply put, If it looked like a duck and walked like a duck and talked like a duck - it must have been a duck. Unless all creation stories were thought be be allegorical, it is unlikely that this one was also.