Still having issues with the duplicator.The workpiece is flexing at the barrel end of the pattern when the follower is on it, allowing the cutter to dig deeper into the practise piece than I wanted.
My older brother (also a tradesman engineer) came up with a suggestion which we then elaborated on.

He suggested a row of holes around each plate, and door bolts to lock them with.

After some changes:
I'll have to get the faceplates remade, but with numbered grooves all the way around the outside of the discs. A sliding plate at each end will lock into these grooves and lock the faceplates together, the numbering will allow it to be indexed together.

Oh joy.
Well. Local clubs Skeet competition tomorrow.