For a safe bake-able "cheap" bulking out you can use any air dry clay or paper mache mix. Make the general shape you need and let dry overnight, at least, then cover with foil to keep the polymer clean. Bake as recommended and when cool you can carve the air dry out. You can get the air dry wet to soften it too.
To help condition a stiff or crumbly clay without a pasta machine sit on it to warm it up. I put it in a storage bag but not sealed air tight. You can also step on it to get the kneading process started.
For arms and legs and other things with tubes wrap a knitting needle with foil then put the clay on. When finished the project easily slides off and is ready for baking. Use a needle the diameter of the string hole you want and you won't need to hollow it out.
Suspend or skewer parts on the pins of a bead baking rack to cure and cool without needing a nest or risk of flattening.