>>159376
>>159363
in theory you'd pour in something like hot wax, let it cool, then pull it out. You'd then take the hardened wax mould of her cavity, and cover it in something with a slightly higher melting point (in the past they'd use this technique for clay, so it'd be clay). Then, you'd heat(fire) the entire thing, and the wax would melt and drip out, giving you a structure that looks and presumably feels like sweetie belle on the inside.
Of course, you'd probably be able to get a more precise mold by pouring clay directly into her and putting her in the oven, then taking the hardened clay and putting plastic around it to form.