Plate steel runs about 65 cents per pound plus shipping. You can have it cut to size usually for a few cents more. Maybe find a machine or fab shop that will get you a piece on their order. You can clean and paint it or even cold blue it to help control rust. Another option is to see if you can find a steel door, i used to have one i got on the cheap because it had a corner crushed and it couldnt be installed.

The old ej lind boards were wire brushed and the grid silk screened on and then lacquered and they held up pretty well.