The circuit boards are mounted on 1/4" hardboard (masonite). You can get the 1/4 sheet from Home Depot (as mentioned). The exact dimensions of my original board are 30 1/16" tall and 18 7/16" wide.
The attached pic shows the mounting of the panel to the door, but this is for the later version with the filter board. Four carriage bolts come through from the outside of the door. The 1/4" hardboard PCB panel goes over these and then a washer and nut holds it together. You can see four mounting bolts in the picture.
Depending on what you are after, a full accurate restoration or just making it functional, you can decide about the foil panel. The foil panel between the door and PCB panel is an EMI shield. There are a couple bolts visible in the picture that are there to accept the grounding straps (one from the cabinet, another from the PCB harness. I can't find the picture now, but there is a pic out there showing the foil panel completely surrounding the circuit boards. You don't see them this way very often (ever). My guess is it was there to meet FCC regs, but then operators removed them to prevent overheating.