Root cells need, like other plant cells, oxygen.
For that your bonsai soil should be well-drained and should have sufficiently large drainage holes in the bottom of the bonsai pot.
Especially with large drainage holes the substrate trickles after repotting from the drainage holes. Therefore, these openings are covered with plastic grids.
How to do that ? Quite simply. Just follow this step by step guide:
It should be slightly larger than the drainage hole of the bonsai pot.
It is sufficient if the wire is about 3-4 times as long as the diameter of the drainage hole. 1.5mm aluminum wire is perfect for this. If the wire is thinner the grid slip easily during potting. If it is too thick, it can be difficult threading through and turn / bow it in the right position.
The width of the U should match the diameter of the drainage hole.