Dura Ace Freehub Compatability - Spacer?

I just bought a set of Cosmic Carbones, and it appears it has a Dura-Ace 10sp freehub. I tried installing a 9sp rear cassette, but the casette wont tighten on it properly as there seems to be about 1-2mm of free play when fully tightened.

Do I have to get a new freehub body, or is there a cheaper solution to the problem? Like a spacer(washer?) I could put on the spoke side of the cassette before installation?

Anyone know???

Thanks!

Andy.