Ok, remember that today many of the available keys are jewelry.
A key is simply a momentary contact switch. The key I learned on was a 6x10 inch pine board with a chunk of tin can and a carpet tack... you file off the bluing so the tack will take solder. No need for details.
But your picture does remind me that you don't have to have a jewelled movement, and you will want a method to vary tension, and that both brass and bronze are considered "self lubricating"... a good feature.
Knowing a machinist, I am sure anything that is "presentable" will work fine.
So start the building!
-Mike.