6 to 8 inches deep? That's not much. The idea is to grow them bigger so give 'em room to grow. If they have developed to your satisfaction, and you want to place them on a tile, then you should also make the box quite a bit bigger than 12x12" because even though you are training the roots to grow laterally , you still need the same root mass to grow the tree larger.

Remember that even a sapling of only a couple of feet tall will have a fairly large root ball.