"Flip the unit over so you can see the top of the case, now you need to find the centre point for you circle, on mine it was quite easy because there was a circular cap which covers the clamp made it easier to find the centre, you will have to work that bit out by yourself.
Once you have done that you need to make the centre circle slightly larger than the clamp to allow for a mounting point for the window. Next get an old CD and place the mark you made to find the centre in the middle of the CD and draw around it..."
For the information on creating your own window, Click
Here.