Hello @619944680,
As mentioned in this previous post, a good starting reference point is reviewing the function _createGeometry which is part of CustomNavCube.js.
In terms of getting the geometry of the cube to your liking, you can look at _createCube which is also part of the aforementioned JavaScript file. Based on the screenshots you provided, your preferred cube shape is a truncated cube which has the corners of the cube bevelled such that the face of the cube is octogonal and the corners being triangular. So you will have to modify _createCube accordingly to create this geometric shape.
Hope this helps,
Tino