It's mentioned in the post you quoted:
But it's really irrelevant what it was used for prior to tearing it apart - the lams of any huge toroidal transformer should work. They just happen to have to correct width and material.
... the lams of a 300W power transformer from an old halogen string light ...
But it's really irrelevant what it was used for prior to tearing it apart - the lams of any huge toroidal transformer should work. They just happen to have to correct width and material.