[captainclock1988]

You should be able to find a 4' x 4 ', or even a 2' x 4', piece of 1/8 inch hardboard (AKA Masonite) at a home supply store like Home Depot. Usually one side is finished smooth. Don't button up the back too tight, you'll want the heat to escape. I did a restoration of a Grundig that had been in a garage for 30 years several years ago and it came out gorgeous. I didn't use any toner, just sealer and clear lacquer.

I think if I were to use a piece of masonite board for making a new back, I would try to drill holes in it following the same or a similar pattern to the original back.