Surprized nobody has mentioned cork!
Go to a Michaels or whatever craft store you have and get some cork craft mats.
Munch em up into small pieces and it's be the lightest bag, but give superior hold and shape.
Also is waterproof - no rice snacks and pea soup on the mound.
I've used it for years in the .22 target rifle genre.
Hope it helps.
All the best, Carl