Polyform has a product called Sculpey Ultra Light, it is very light and fluffy very much like Hearty or LaDoll Premier air-dry clays only it's a PMC that requires baking.
It was actually very nice to sculpt with but VERY soft. When I boiled the first time everything went great, but the second layer/boil boiled off. There was cured fluffy crap floating everywhere. When I took the sculpture out of the water the original cured part crumbled away like ash.