I'm sure there's a number of ways you could do it, but the easiest to do (probably) would be just "standard" treadmill/conveyer belt material... I don't know what it's called but it's almost always the same kinda stuff -- this black cloth/hard rubber material covered in tiny divots for traction.
This kinda stuff:
http://www.made-in-china.com/image/2...-Polishing.jpg
You could always get more complex and do a series of thinner belts, use cloth, etc, but your best bet's probably just that "standard" material imo. Seemed to work pretty well on our '06 bot.