The grow box should be fine if it is deep enough (~ 6"). I treat deodar cedar similar to pines when it comes to initial potting and repotting. They do not have fine roots. Very coarse and thick, also, sensitive to severe root work. Need a fairly loose well draining mix that dries and is not too water retentive. They like to be dry, uncomfortably dry… As a bonsai practitioner I am conditioned to water at least once per day and usually between 2-3 times on average. I have learned the hard way and now care for my deodar with great neglect. They do much, much better! I almost never water it and its scary how well it does simply by being neglected. I have also learned they need full sun, and only full sun. They slowly decline in otherwise, with browning needles or foilage pads, then a branch or two. After this the decline is rapid. If healthy they seem to have an intermediate growth in terms of rate of branch and needle elongation. Ive noticed growth is in latter spring and well into summer. I have not seen any appreciable trunk thickening of my sole surving deodar, and its been under my care (neglect rather) for almost 4 years. It and the others, bless their little bonsai souls have been finicky and I have yet to begin anything past setting structural branches. They seem very flexible and use of raffia is my recommendation… experience is the greatest teacher of all. If you need to make large cuts… Expect it to be part of final design as a Jin or possibly Shari.
I am by no means an authority on deodar, just relaying what little insight I have.Despite all the struggle I keep hope that one day it will persevere and become a glorious bonsai specimen. I just let it tell me what it likes and have given it free reign of its own destiny. Good luck with your specimen!