I just use ordinary 1/2" vinyl insulation tape. If I need a thinner strip I'll cut it down on a cutting board. I also cut the tape length to match the circumference of the piece, so there's no overlap. Sometimes it's a mm or two short so I make sure the gap is not under the chuck jaws. I've found that if you very carefully do the chuck up the alloy won't mark as the work often doesn't need to be mounted that tightly. To my surprise the work normally centres up really nicely, it's only when the piece is overtightened that the tape compresses too much and things go horribly wrong.
This week I found some soft jaws available for small chucks - anyone use them, and are they any good?