Making the Template
Next step is to make a template for your new mesh. I did this buy getting a marker, in this case purple (you'll see why later), and colouring the outer edge on the back of the grille.
Quickly press the card/paper onto the back of the grille, if you do this quick enough and you will have a rough outline of the back of the grille on the card/paper to follow. This will allow you to get roughly the right size and shape for future cutting.
To make my life easier I then cut this out.