The exposure (or discharge) plate is made from 4" x 6" single sided copper clad board. The high voltage output wire (green) from transformer T1 is soldered to the corner of the copper clad board. The copper side of the board is placed face up and glued to the wood board.
After the glue dries, the discharge plate is covered with a thin plastic sheet approximately .005 inch thick. The plastic should extend at least 1" past the discharge plate on all sides. The sheet is secured in place by taping or gluing it down.
The thickness of the plastic sheet isn't critical. Anything around this thickness that will prevent the object from making direct contact with the discharge plate. Sheets are available from many art and drafting supply stores. The plastic sheet may be transparent or opaque, it doesn't matter.
To test the circuit, attach a wire to point A shown on the schematic and place the other wire about ¼" away from the discharge plate or the uninsulated end of the high voltage wire. Note the discharge plate should not have the plastic sheet covering it during the test. Every time you flick the switch SW1 you should see a spark jump from the HV wire to the discharge plate.
When the circuit checks out properly, mount the components on the board using glue or epoxy.
Coat any exposed wires with a plastic spray to provide insulation (No-Arc spray is available at your local Radio-Shack stores.)
Cover discharge plate with plastic sheet.