Light sculpting material (preferably air drying?)

[Folly]

I'm working on customizing a Monster High doll.  Said custom involves turning her arms into tentacles, and I'm finding it nearly impossible to make them long enough (we're talking knee-length if I want perfect accuracy) without weighing down on the arm joint... I don't mind it being a little loose, but I'd prefer if it was at least posable :<
I'm using milliput since that's what I have on hand, but is there something that weighs less I can use?  Or a trick to make them lighter?  And, semi-related, would baking an MH arm melt it?  It seems like most of the clay that boasts being lightweight, you have to bake

[Flaileigh]

Try Creative paper clay. It's an air drying clay that you can get a Micheal's. I used it to to sculpt hair on Barbie's before. 

http://www.paperclay.com/

If you haven't yet, make the tentacles out of wire first, attach them to the arm and then cover them in the clay. It'll add to the stability of the tentacle and uses less clay.

[Rainsong]

Armatures. Wire or even aluminum (or both!) in the middle with a thinner layer of clay to make the rest. Makes it lighter as well as having an easier time making/keeping shape, especially odd ones like curliques or rounded ends like tentacles might have, since heavier hunks of clay want to sag or shift with their own weight. You'd just use the wire as a form, and put aluminum over it to fill it out/add bulk, then put clay over that.

Pretty sure I've read about people baking MH arms/legs/bodies without a problem. Fimo clay only needs 230 degrees F by my package, which is fairly low- Sculpey was something like 275 by my roommate's explanation the other night.

[aerieberry]

Some milliput on top of a bit of wire+alluminum will do the trick! <3

[Alicat]

you could also check into the light weight Sculpy. I got mine at Hobby Lobby. I had to make large horns( 8 inches-ish) for my head that would be on for 10 hours a day, and it worked great!

I did begin with an aluminum foil armature, as stated above.