While I'm not (yet) prepared to accept plastic, totally natural amber is not necessarily a requirement for this. After all, the amber is itself a substitute for the quartz crystal in the originals- I foolishly thought it'd be easier to find. Ha! :)
So. If anyone can point me toward either amber or clear(ish) quartz round cabochons in the 1-1.5 inch (23-35mm) range, I would be very, very obliged. Just sayin'!