Steel plate works fine...
I bought a piece of "scrap" steel plate from the Steel Yard on Columbia Blvd for a very reasonable price. It is about 1/2 inch thick and about 20" x 30". It took some effort with a belt sander with progressively finer grades of sandpaper to smooth and polish the irregularities on one side to get a nice smooth surface for back riveting. Works very well. I built two RV-8s with it. I am not sure that you need a back-riveting plate any larger than mine. Mine is heavy but still portable if I need to move it; anything much larger would have to stay in one place.
Dan Miller
RV-8 N3TU 730 hours