to begin, let's draw a rectangle on the canvas.  This will serve as the core to hold the yarn as we wrap it around.

To eliminate any sharp edges, let's add in the square shape with rounded edges.  We will resize these for the ends of the design.

Next, let's unite the ends with the core to create the design.  

Now let's add in some cutouts that allow us to tie off the end or simply slide the string down into the slot.  This will keep the roll of yarn from unraveling. To do this, simply add two more rectangles and subtract these from the top of the design. 

These settings are for the D1 with a 5W laser.  Be sure to follow the safety rules for your laser and always wear personal protective equipment when the laser is in operation.