I would just get two pillow inserts (aka, pillow forms) and use those. It would be much easier and neater than trying to do something with loose stuffing, batting or foam. A standard lumbar form is 16x20 and is firm. Another standard size is 12x24, but remember that, in a pillow, 12 inches will appear much narrower due to the rounding of the surface. Might wind up looking a bit like a pencil! You can also get custom made sizes from some places online. Forms can be filled with down, polyfill, foam, or a combination of those, for varying degrees of firmness. Lots of places online to find them (I've gotten some great ones from The Company Store when they go on sale).