Blend Curves

Draw a rectangle 450 x 380

Select the rectangle

Hover on the top rectangular grip
Select Convert to Arc

Move cursor up and enter 70

Modify panel > Fillet

Command line: Radius

Specify fillet radius: 70

Command line: Multiple

Select 2 lines at the left corner

Select 2 lines at the right corner

Modify panel > Explode

Select the object

Modify panel > Offset

Enter 50

Select the top arc

Pick a point on top

Modify pabel > Blend Curves

Select the top arc

Select the bottom arc

Repeat the same steps on the right arcs