Starting with a marker pen, a razor blade/hobby knife and large sheets of paper or several A3 sheets taped together to make a suitably larger piece, I 'formed' the sheets into the appropriate shape, making sure that there were not too many violent curves or awkward-to-adhere shapes used.

Once happy with the general shape, I marked the insulating material on the silver side (else a mirror image was made) and cut it a little larger than the markings. A couple of sharp blades were required, as the aluminium matting blunted the cutting edge of the mat knife rather frequently.