The original is compressed, un-corrugated cardboard, 1/8" thick.

At Home Depot or Lowe's you can find in the finished lumber/molding aisle about 4 non-plywood items 1/8" x 48" x 96", some of which are pre-finished white, some not. You might like one of the non-white items. With a coat of matte black on it, the only way to tell it is non-original is to remove it and examine it closely.