[bardd]

We used smooth aluminum that was the inside of a c-channel (well, two c-channels), I don't remember which type.
We didn't do any tests, but I see no reason not to go with the smoothest material you can find, as long as you know it won't break (which I doubt will happen with any but the flimsiest of materials).
Since the disc touches it all the way through its lip (or at least on the sides, but it'll almost certainly be both sides), so the friction between the disc and the "floor" doesn't change the spin, only slows the disc down, so you want as little of it as possible.