Use poster board, they sell them at Walmarts and more choices at craft and hobby stores (various thickness, surface texture, colors.)

Use a jig saw to cut a bit of wood into the contour you're looking for.

Staple or nail poster board to the wood.

If nails or staples are obvious, cover them up with paint or other substance, or make them look like rivets.

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*~~~{==========- Death at Midnight -==========}~~~*