How tricky would this be to machine? I guess a Triac is not up to the job!

[magicniner]

Have you considered machining a pair of aluminium soft jaws for your mill vice? It's a great way to accurately hold oddly shaped parts in the mill.
Machine a pocket across the two jaws that comfortably accepts and supports the part, use a parallel as a spacer to allow you to clamp the vice up tightly with the right gap whilst machining the soft jaws.

- Nick