I'd invest in a wooden packing crate big enough to permit some straw inderneath your tree. I would probably repot into a smaller (5") daffodil paper-white pan, by simply potting up the soil ball with added soil around the existing soil.
*Soil* its not what you think it is.
For a single tree I would probably buy some cactus mix from one of the big-box stores. its not going to look a bit like the soiless-mix you are trying to reach for. Soiless-mixes and potting soils have too fine particles. Your tree will drown (and asphyxiate) in potting soil.
Now that your tree baby is in a plastic pot and nested in a bigger box on the porch, you are still going to depend on touch for need of water.
Its not *if* your tree baby freezes, but how many times it freezes and defrosts. better it freezes in December, and stays froze till March. You are still going to poke it with a chop-stick or finger daily.
A frozen tree tends to be brittle. It needs protection from fido or Kitty walking on it.
Hope this helps.
Your next history lesson: The Larz Anderson bonsai collection was exhibited for many years at the Boston Flower show. It and the bronx-zoo collection were old collections of retired diplomatic corps members.
https://en.wikipedia.org/wiki/Larz_Ander ... Collection may have leads to how they store their trees.