Phenolic or capped melamine imo.
Truth be known....Ivorine 4 is the strongest ferrule material ever offered. It'll flatten hacksaw teeth in about 10 secs when trying to cut small ferrule rod. It takes carbide to cut it. You can use it in any fashion you like too as it needs not be capped. :wink:
What about a Carbon Fiber Ferrule? I have access to some thru work.