Actually, just checked custom design site and its no longer aluminium. the following is a description of what they now have
"The Non Magnetic Stainless Steel Inert Casework Matt is placed on top of your equipment and absorbs unwanted resonance which occurs in the case work of CD players, DVD players and Amps. The Matt is manufactured from the now famous Acoustic Polymer with stainless steel which Custom Design has pioneered. The system works on the principle of constrained layer dampening, which is two sheets of the same material of different thicknesses, a ratio of 3:1 we have found to be the most efficient, bonded together with the patented polymer. The polymers viscoelastic properties converts kinetic energy to heat, by preventing excessive resonance from effecting the performance of your equipment the Inert Casework Matt will enhance the sound by providing clarity, separation and adding an inky blackness to your base. It will also assist in producing sharper imaging and vocals for DVD players."