Create a new ellipse to the size shown above and center the ellipse over the bottom of the box shape. Overlap the top of the ellipse so there is space on either side. Select both and Arrange > Combine Shapes > Subtract Shapes or press Ctrl 2.
Switch to the Shape Tool (F4). Click on the outline of the box shape as shown above and add two new control points. (I dragged a guideline from the top screen ruler (Ctrl L to display rulers) to the top of the bottom arc to help position the two new points).
Select the bottom two corner control points and delete them.
Clone the shape (Ctrl k). A clone is a copy created in the same position.
Select the Straight Line Tool. Click and drag to add two diagonal vertical lines as shown above. The top of the lines should pass about 10 pixels away from the edges of the top arc and extend beyond the shape.
Hold down the Shift key (to select multiple objects) and click one of the lines then click the clone shape and Arrange > Combine Shapes > Slice Shapes or press Ctrl 4. Repeat this for the shape and the other line. Select the center portion of the cloned shape and delete it.
