I remember life jackets filled with kapok

virtues of kapok
 

Kapok is a common stuffing for meditation cushions, as it is "firm but buoyant" and pretty lightweight. It would be good for any kind of pillows to sit on, as you would stuff it densely and so not have lumps. If the pillow gets flattened, putting it in the sun (or probably just warming it well) puffs up the fibers. (Quote is from stillsitting.com)