cut along where the yellow lines are indicated on the image below. take care when drilling and sawing to only cut the plastic box and nothing beneath. you will be fairly safe if you cut only where the yellow lines indicate. once you have cut along the top of the box, use a broad screwdriver to remove the cut section, you may have to lever and snap away a small section of plastic that remains from either side of the top section so that it can be lifted away. it should now look like this.
now you will need to cut a small section from the right of the grey dash 