It's a piece of grey plastic about a 1/8" thick.
(It is actually meant as a mouse pad, it's for like drafters or something...it's got measurements and stuff on it, I don't remember where I got it)
It's perfectly flat, and it doesn't attract dust like those hideous cloth things.
It's not as good as 3M PMS, but it works. I'm planning on buying an optical soon then I'll probably use nothing.