There's a lightweight "clay" that you can sculpt with. Look for it on the isle of a craft store with sculpy, but it's fast drying (abou t24 hours air drying), with a foam-like consistancy. You don't bake it, it's fragile, but it's also super-easy to use and super light.
There's also resin, but that would take more experience to get right, I think.
(no subject)
There's also resin, but that would take more experience to get right, I think.