It seems to be pretty obvious to me, Joan. You seem to be answering your own question. I'm not experienced in this. But, it appears that no one is willing to give you a reply. So, here goes my two cents worth. I'd use 1/4" masonite, sandwiched with an edging of 1"x 2" s, primed with a gesso on both sides. You can add a moulding to the front side to dress it up, then paint it as you see fit. Masonite and plywood will weigh approximately the same. So, the choice would be a toss up as to what to use. True, it will have some weight. But, it will need that weight to be stable.
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Regards, Tom
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