We had some friends visiting in a truck years ago, and in addition to all of the above, when they were getting set to leave I took a few 2x4s and whacked together a simple frame for in the bed. A 2x4 in front and another behind the wheel-wells, and a pair of 2x4s in-between. Then put the kitty-litter / sand in the rectangle formed by the frame. That keeps it from sliding around in the bed.