I do not think an adapter wiil work with those older round, plexiglass Apple speakers.
If they did, many Apple users (myself included) would've kept using these speakers on newer Macs.
It is not just the diameter of the jack on those speakers that is the issue, it's the actual length of the jack that is the real issue.
The length of the jack is very short and, as usual for Apple at that time, a non-standard length.
That is the real problem. Apple made sure these speakers only worked with Apple computers of the day.
They were designed to be proprietary and you could not use these speakers with anything else, but specific Apple computers.
And why no Apple user who has these speakers have not been able to use these on later Mac models.
They were specifically design to use with the G4 iMac and other G4 models of the time.
These speakers work with my 2003 G4 tower, but can't use with my much newer 2009 iMac.