Draw a rectangle by entering the command Rectangle; select the first corner point and then type in the dimensions.

Click on Points on to enable shape editing.

Select the both the control points and drag them down using the Gumball.

Type in Mirror Command and select the right bottom corner as the start of the mirror plane.

Hold shift to turn on Ortho and drag the plane towards the top, to mirror the shape.

Click on Perspective, hold on to the mouse's right button and drag the mouse to rotate the view.

Click on Points on, then click and drag the mouse over the common edge.

Drag the edge ahead using the Gumball.

Use the mirror command as described before to create a mirror image of the transformed shape

Type in the Polyline command, then select the corner of the central edge as the start point and mid point of the opposite edge as the end point.

Use trim command to delete the extra length of the polyline, ahead of the intersection.

Rotate the model 180° and connect the intersection of the edges.

To form surfaces along the planar curves, type in the command PlanarSrf and select the curves

Repeat the same process for other planar curves.

Repeat the same command for other planar lines. When repeating  command just press the space bar or right mouse button, no need to type the command again.

Input the command SelCrv, this will select all the curves and then press delete to remove them.

Input the command Join to connect all the surfaces together.

Type in the command Copy and select the form.

Click on the form and drag it away.

Select the copied from, hold on to the red curve on the gumball and move the mouse to rotate.

Now select the form and type in M to move it. Select the corner of the top edge and then drag it to the corner of the top edge of the original form.

Rotate the view to see the top surface. Enter the Mirror command and define the mirror plane along the inner surface.

To repeat the module use the Copy command.

Keep copying the module to achieve a linear form.

Now copy the linear form along the shorter axis.

Modules in the end and middle were deleted to break the rhythm. The final form can be viewed with various types of rendering. Here the artistic view is utilized to enhance the visibility of the internal corners.