I spent many years shooting food groups with a chunk of 6x6, a short 2x4, and various sand bags from sleeves and pant legs. They all worked well but I learned if the sandbag itself was tubular in some sort, I preferred a handful of smaller bags than one or two bigger bags to build up my benchrest position.
I quit messing with them a few years ago and bought a filled Caldwell set on sale locally. Decent bag up front and a “rabbit ear” bag for under the butt stock is pretty trouble free and very easy to use. I don’t see that they will wear out anytime soon, so it’s a small investment that will last for many years.
Still, if you want cheap and easy, buy a package of tube socks. Fill with desired media, plastic beads or bb, sand, rice, etc and tie the open end tight and you have enough to build a very solid shooting platform for both the front and rear of your rifle.