Shield windings (a winding between primary and secondary with one end connected to a “quiet” primary side node and the other left unconnected) are a somewhat common technique. Simply wrapping a sheet of copper around the bobbin between primary and secondary will act as a shorted turn, and give less than desirable (though perhaps spectacular) results.
I’ll disclaim the title of “expert” but do have some experience in areas of EMI mitigation and magnetics design. The private message function can be used to send contact info or more detailed information that’s not suitable for a public forum.
