(a.) Select the V shape (you did keep it didn't you?) and make it slightly wider than the width of the cube. Center it so it is halfway down as shown. (b.) Place a copy of the V shape in the clipboard (Ctrl-C). Select the V-shaped line then the left side and Combine Shapes -- Subtract Shapes (Arrange menu). The V-shaped line disappears. With the side still selected, Break Shapes (Arrange menu) to separate the side into two panels. Paste a copy of the V-shaped line directly on top (Shift -- Ctrl V) and repeat the process to separate the other side.
(c.) Make a duplicate of the top of the cube. Set the fill to none. Make another duplicate and enlarge the duplicate 240%.