Plywood as a temporary material will do. But if you want to make a thorough and age, you need a thick cardboard. Who is glued to the frame (frame is rigid). On his turn, glued work. Board does not crack over time, unlike plywood. A frame provides the necessary rigidity. This is called "English stretcher. It is used usually for graphics. In this case, I think, too, is permissible.
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