This isn't strictly knitting, but -- making a dress form that exactly fits you is one of those things that's handy no matter what. You could use duct tape, but paper tape is easier to put pins into later, and leaves less sticky residue.
Use expanding foam from the hardware store to stuff the dress-form.That stuff hardens well enough you will never have to re-stuff it, but still ok with pins, etc. To lessen volume, first stuff with packing peanuts or crumpled newspaper. Be careful to keep the body doubles' shape the same while the foam expands. Working with small sections at a time should help.