I have to admit it, sand sock or sandbag, both my benches have a couple healthy sandbags on them, and both have a bag of lead shot on them.
*IF* you are going to carry that bag around very much, consider something like 'AirSoft' plastic pellets instead of something really heavy.
Only one has a couple different kinds of (mechanical) rests, which I rarely use (the reason they are rusting).
'Bench' shooters wouldn't know anything about this
I learned from military field shooting, stuff your bags with beans or rice,
When the mission goes long you still have food!
And the rifle doesn't know the difference between sand & rice,
and the 'Sock' doesn't care there is a plastic bag in there!
Beans & rice are lighter than sand, ounces equal pounds & pounds equal pain for field shooters.
Old time shooters used to use 'Hard Rice', kind of a varnish on the rice, don't know if it was done by some manufacturer or grew that way naturally, lighter than sand or shot and worked just as well without rotting.
About 35 years ago a military rigger made me some 'Sand Socks' out of heavy nylon canvas, about 4" in diameter and 5" long, strap across one end, I still use them, handy as all get out! I didn't know at the time they would be this handy this many years down the road...
One of the LEATHER bag companies makes a stiff, heavy leather version of that little bag, round & overstuffed it doesn't quite settle the rifle in like the nylon version does...