BEWARE, adding anything to your computer MAY void the warranty. proceed at your own risk. I have not brought my computer to apple after adding the button so I dont know if they will care that i added the button, and even if they said anything, the button pops right off. But be careful.
Also, I have tried other materials, but most allow the transfer of gestures from your thumb, through the material, into the trackpad. The plastic from the DVD case really works the best!