Along with swing and crafting I also do home improvements. There is a product available at the hardware store that would be perfect for filling a custom dress form. It is pressurised spray foam insulation. It comes in a can about the size of a typical spray paint can. You would need to make the foam as before and make sure the tape around the cardboard at the bottom and arms along with the back is secure. It would probably help to follow the suggestion regarding the dowels through the middle. Then just spray the foam in from the top. It will take at least a day to dry but will be a hard form that shouldn't loose shape. I'm going to give this a try over the weekend and will post again with the results.