I have adjustable shelves, so 20X30 inch foam core was perfect, and I used the full width, got four 7 1/2 X 20 inch pieces per board. Now I wish I had made them 15 X20 inches - would have more fabrics on each shelf. I bought most of the foam core on sale, 2 sheets for $1.
I can hear you saying, "How did you fold the fabric to fit?' The fabric was 1 or 2 inches taller than the foam core, but when all the fabric is on a shelf, the part that's not on the foam core is supported by the fabric 'next-door' etc etc all along the shelf, so it doesn't matter that the foam core isn't quite tall enough.