I went back and took a look at th diagram I posted a year ago of an alternate method of securing the roots on a board.
Actually, I think this is illustrating using a tile, rather than a board:
View attachment 106613
Upon closer inspection, I noticed that the tree was not attached with a screw. But rather, held in place by wires run thru the inner square. It appears that holes were drilled thru the tile to allow the wire to pass thru. Then, when potted, since the roots were tiny, they resorted to tying the trunk in place with string run around and under the pot. See Fig 14.
Then, later, when the roots began to grow, they needed protection from the wire so that they would not cut in. See Fig 16.
So, which way would I prefer? I think the original way with a board works just great. The tile requires drilling, you cannot direct the roots with nails (or screws) you have to provide string to secure the trunk, and you have to make sure the wire doesn't cut in. The advantage is the tile wouldn't rot, and when the tree is "done" you can use the tile on another one. Wood boards need replacing from time to time.