well if i took something like bed foam and sew it and cut it into large clam like pillow shapes i could insert a large ball or pillow the size of my body to hold the shape inside then sew it closed. then coat the shell in layers of a hard coating material so the clam holds its shape around the ball or pillow, then you could seam rip or cut the clam open so you have your separate halves, and now because the outside has been stretched and bent to fit over a round body like shape then outside of the shell will keep its form... so now you just coat the inside halves in layers.
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