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perhaps you could use the cone from modern speaker? if the donor was just a little larger you might be able to use that.
you can make a cone from light construction paper. it should be a thin snappy paper.

The easiest way is to see if a fellow radio collector has a spare speaker. If you measured the diameter, and check the number of ohms across the field coil and the voice coil. ( mainly the field coil , the voice coil is not many turns) close will usually work good enough to make it work.
It wouldn’t hurt to put the area that you are writing in from in your profile. That could allow others near you to help better.