Or cover a mannequin with a towel and just heat mould the foam over it. Heat gun powrrrrs activate!

The other option for the dress is crepe. That shit is seriously underestimated, it drapes beautifully. It's what real Japanese kimonos are made from. You can get it in silk or faux stuff called crepe de Chine. It can be quite thin so you'd need to line it anyway.