Besides using twine, rope, wire, I’ve noticed my local niwaki/bonsai nursery also uses a lot of small 2.5” plastic pots filled w concrete and a hook.
I have no concrete tho; but I do have a lot of landscape pebbles…so, I made some weights from pebbles.
Looks way better than a plastic pot filled with concrete too.
Advantages: Super quick and easy to use, compared to tying knots with twine/rope or wrapping with wire. And reusable.