There are about three things that this very famous passage, 1 Kings 7 implies about the temple's large laver.
First note that the vessel was 10 cubits across and 5 cubits deep - this it was (in modern terms) hemispherical. This may explain the comment, "round all round" not simply circular, but this is not essential. In any case, the top rim was circular. More likely, the shape was approximately cylindrical, and 5 cubits deep.
Ancient measurements of length were nowhere near as accurate as modern ones. Errors of a few percent were common. If our modern ratio of "pi" (= 3.1415926535 …) were approximated by just 3, then this represents an error of less than 5% - a lot even for the ancients. However, as shown below, such an error is not even necessary to assume.
3. Which Circumference?
The large bronze basin would have had a finite thickness of many inches. Just how thick is suggested in the same text as about one hand breadth (1 Kings 7:26), that is about 3 to 4 inches thick.
Now, let us suppose (for the present discussion) that a cubit is approximately 18 inches in length (whether a little more or less does not alter the argument below). If we assume that 10 cubits (180 inches) was the outside diameter and 30 cubits (15 feet = 540 inches) was the inside circumference (as would be expected from using a straight edge), then this would represent a thickness of just 4 inches!!
Thus, I see no problem here at all.
APPENDIX - Volume
If we assume, as is more likely, that the ancient cubit was closer to 19.5 inches (= 0.5 meters), then we get some very interesting results for the volume of the water contained by the large laver. For a cylindrical laver, the internal volume works out to be about 44,000 liters or about 2000 Hebrew baths as per v26. This confirms the other measurement in v26 and also confirms that the 30 cubits is the inside measurement of the circumference and thus, 10 cubits was the outer diameter.