What can be left inside sculpey polymer clay to both support its shape and not burn up when fired in a stove?

I’m looking for something I can use to make a box like shape that will keep the polymer clay from sagging before and during fireung. I considered Using poster board to hold it in place and let it dry before removing it for the fire but since polymer clay never dries I would need something that would stay inside and... show more I’m looking for something I can use to make a box like shape that will keep the polymer clay from sagging before and during fireung. I considered Using poster board to hold it in place and let it dry before removing it for the fire but since polymer clay never dries I would need something that would stay inside and maybe supported after it’s dry as a bone.
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