[ChrisH]

If you can't find a mill, mark the aluminum plate carefully with the octogon and cut it out with a band saw. Leave it slightly oversize and clean up the sides with a file. It does not have to be all that precise to work.

You might also be able to get the part that mates to the white "crown" from a place that sells FP parts. The part is called a "jeep driver" and is pretty inexpensive. The jeep driver makes a pretty good drum by itself and, at least with some year's motors, was just about exactly the right size to make a drum for a robot lifting winch. The jeep driver used to be in the KOP so you might find a picture if you look though the old docs.