Schmelke still has them on their website. They appear to be aluminum. Can't confirm because their site is terrible.Why does no one on the planet earth carry or make a "Uni-loc shaft to 5/16 18 butt Joint adapter" or conversion piece. Schmelke use to make them. What the heck happened. Just no market for the product? Sure, they carry joint adapters in just about every other thread. Doesn't make sense.
In a short web search, I can't find one that isn't aluminum. I figure aluminum would be fine unless you are worried about your butt/shaft in conjunction with the adapter.
If you want something else like wood or CF, you would probably have to get a cue maker to custom fit one for you. There was a discussion about something like this on AZB recently if you care to look.
It would be interesting to see a company like BalanceRite to make adapters out of their extensions.