I would use either paperclay or wonderflex.
There's a tutorial somewhere wbout making a cloud one out of wonderflex, I'll edit when I find it.
My KH cloud armor is paperclay and it's super light.
You would oviously sand the paperclay and gesso more then I did, but I went for a used look..
What I did:
get a wig head or other rounded object close to your shoulder size.
Buy paperclay, something to put inbetween the round item and the clay, small bowl with some water in it and a towel.
Gesso (walmart is a better buy then craft stores) and black paint.
If you use a wig head you can put fabric as the item between the wig head and clay (I liearned the hard way it sticks to the head without fabric).
Then using small pieces of clay make it thin like you would with play-doh and put it all together however you want and into a rounded shape.
Then let it dray. It will shrink so make it a little bigger then you want it..
After it dries, use more clay to fill in the cracks, (do not use it all in one time, you will need maybe 3 bags to finish both armors), after dry take it off the round thing. there will be cracks inside too..You no longer need the round thing..
Then, after you got it all filled and dried and there's no more craks or weak spots sand she shit out of it. A mouse sander will work better (electric) but hand sanding is more presice.., then cover it in gesso and let it dry. Sand the gesso, repeat a few more times intil it's nice and covered yet sanded.
Next paint. Use maybe 2-4 coats..then bam add them to the straps..You can use a few different ways..If you want to bolt them to the straps, then when making it at the start, make sure theres big enough hols to get the bolts through, once dry you can;t do that..also make sure the boltet areas are pretty thick because you want them extra strong.
Hope this helps..
I will edit with the wonderflex tut in a little bit, I have to go for now. If someone else posts it then nvm..
This is the wonderflex thing I mentioned..