The people of Ether were instructed of the Lord to build boats that were round also and left shortly after the Tower of Babel was build and the languages were changed.

Gen. 6: 14.
14 ¶ Make thee an aark of gopher wood; brooms shalt thou make in the ark, and shalt pitch it within and without with pitch.

Ether 2: 17.
17 And they were built after a manner that they were exceedingly atight, even that they would hold water like unto a dish; and the bottom thereof was tight like unto a dish; and the sides thereof were tight like unto a dish; and the ends thereof were peaked; and the top thereof was tight like unto a dish; and the length thereof was the length of a tree; and the door thereof, when it was shut, was tight like unto a dish.
Ether 6:7
7 And it came to pass that when they were buried in the deep there was no water that could hurt them, their vessels being atight like unto a dish, and also they were tight like unto the bark of NOAH; therefore when they were encompassed about by many waters they did cry unto the Lord, and he did bring them forth again upon the top of the waters.

057: One iku (about 3600 square metres or 60 x 60 meters) was its floor space, twenty cubits each (10 meters), was the height of its walls,

058: One hundred and twenty cubits (about 60 metres) measured each side of its deck,

Furthermore, the Magurgur was not intended just to float and not move, according to Finkel (allegedly), because it started at Shurupak the hometown of the Sumerian Ziusudra (Ut-Napishtim in the Akkadian recension) and ended at Dilmun in the Gulf, some 280 miles south (lines 139 and 140 of tablet XI):

139: At a distance of fourteen double-hours (about 280 miles), there emerged a stretch of sand bank,

140: On Dilmun (Bahrain) the ship was held fast.

Finally, there was no such a king in Sumer or anywhere else in ancient Iraq, either before or after the flood of 3050 BC, called “Atrahasis”. This is a phrase meaning “very wise”, not a proper name but an adjective applied to Ziusudra in some of the translations of the Epic of Gilgamish.

Getting to grips with ancient (or even modern) history is an exceedingly difficult exercise, not helped by being infected with falsehoods, either honestly or dishonestly.

When you say the Mesopotamian ark is a cube, do you mean there are specific reference to shape and dimensions in other tablets, or that the “Draw out the boat that you will built with a circular design” quotation is a kind of template for making a cube, like the Vitruvian Man stretching out in a square inside a circle?