Any toolsteel hardened and tempered rings pretty good I just experemented till I got the sound I was looking for. I have used the inside of some polished rods out of some big cylinders that made some good sound and are holding up pretty good for wear to, but any toolsteel should too. That is what I've done so far for bigger rowels, smaller rowels were the sound doesn't matter that much and things dont have a tendency of bending that much a mild steel with a little hardness works plum fine. I guess whatever HOLDS UP WELL and sounds the way you want and sells is the ticket the way I see it, everyone has a different veiw.
Willey