I often create my own "throwaways" using either rice or small white (Dad calls 'em Navy) Beans - the kind that work well as bullets for straw puff N shoot weapons :-)
Whatever size, I like the larger ones, ZipLoc bags do the trick for your camera(s). Small pillow cases, or a heavier material if you prefer - I've used burlap with some success - and clear packing tape. Or, better yet, gaffer's tape.
When you use rice or dried beans, and keep them sealed inside a dry ZipLoc, zipped closed, then taped on each side - yeah, I know, but the zip side is actually tougher than the three other seams, so I reinforce ALL four sides with clear packing tape. Just before sealing the zip, push out all the air you possibly can. Here, I use a pillow, pushing down on the bag lying on a hard counter surface, expelling as much air as possible before zipping and taping.
NOTE: you can, of course, use those plastic or foam pellets, but I like a bit more bulk to my bags even though they are a bit heavier. They don't seem to "sift" or "shift" as much as some of the lighter-weight materials sometimes do.
I've even found "sort-of" carpet material type zippered cases like replace covers on various-sized throw pillows for sofas, chairs, etc. They work nice as well, and are easy to close at the open end after inserting the bean/rice ZipLoc bag. Otherwise, I am often able to get two bags out of a standard sized pillow case. Or I make my own cases (the burlap) sewing them closed (usually by hand) at the opened end(s).
They work very well, are handy and a few can be made up during one of your "down" times. And, provided the beans or rice remain dry, you can reuse them for replacement bags, or if your grocery budget is taking a hit, pour them into some boiling water and eat the things - well, the rice and beans, I don't recommend eating the other stuff. Toss and replace at will.
