How can I add a shielding screen between the primary and secondary in order to reduce capacitance?
To specify an electrostatic shield, you can use a 1 turn foil winding with the appropriate thickness of wrapper insulation and end margins to satisfy the voltage breakdown requirements between the windings and the shield. You must initially specify a voltage and current; then after the initial design has been made you can edit the turns and wire size to achieve the desired results.