The experiment in bonding seems to have worked. After almost 18 hours of cure time - not strictly required but entirely due to my paranoia - the mounting is solid. Will it last with the weight of the stepper motor on it, and with general handling? That remains to be seen. All seems well so far, at least.
Here is a photograph of the mounting to show the three 120 degree spaced holes. These are 5.1mm, to suit the M5 attachment screws:-
And here is the part fitted to the rotary table:-
The next task will be to drill the mounting holes for the stepper motor in the square portion, then mill the coupling access slot. After that it will be time to assemble the electronics in the die cast case. Keep watching.