I have used sand where I need volume and weight. However I use Caldwell shooting rests and I put lead shot bags on those for weight. I think a good masonry sand will be best if you want volume and weight. Hope this helps.
I've had poly beads in bags that are 20 years old and still hold up like brand new. They are impervious to moisture. You can find them at a local hobby arts and crafts store but here's a link to Amazon.