Whether it says 'completely round' or not depends on
your translation.. More importantly it is not at all certain that the description it gives is the diameter and circumference of the same circle, especially given the following passage where it says "It was a handbreadth thick; and its brim was shaped like the brim of a cup, like a lily blossom" (from the NKJV translation you quoted).
As rare as it may be, I think Answers in Genesis gets
this one right (the 2nd suggestion, not the 1st).