Robert had his made for him I believe from High Density Polyethylene (HDPE) . Not positive. You can buy this in small sheets at different thicknesses.
http://www.usplastic.com/catalog/ite...ickid=redirect
You can also buy spacers at various thicknesses.
These are just one example. http://www.sonicelectronix.com/item_...a-82-4300.html
Now the thing about cutting your own is that you can make it exactly as you need and the High density plastic will be much more inert/solid.