According to Synergistic research aluminum is the material of choice.
these aren't on the cheap: but Money back guarantee- I didn't hear a difference on my macbook with my $2 dollar speakers and $10 DAC :)
But this would be an easy blind test- for those of you whose computer is hooked up to your hifi- have someone else hide the computer from view, and have them click the video back and forth on the with/without risers sections and see if you can tell the difference 9/10 times.