Anderson deep propagation trays
They are marketed as propagation trays, but work great for developing nebari and roots.
Some variations on the strategy:
1. Line the bottom of the tray with "weed barrier".
2. No lining. We find this gives us the best root development (see the attached pictures). Caution: the roots will grow vigorously into the ground if you leave the tray on the ground. Put down some cover or weed barrier.
3. Line the bottom of the tray with newspaper. This provides a temporary barrier.
4. Put your tree on a tile, and then in the tray.
Have fun,
Chris