personally i dont know nothing about anything but i think that getting in touch with the/a builder that you have in mind to do the work would be a great start to answering your question.
they ship all of the time and they will be able to give you a true answer.
I use a cardboard box large enough for the accordion and 4 to 6 inches of foam padding on all sides.
An improvement on that would be to double box it. Inner box large enough to hold the accordion snuggly, with some padding. Outer box large enough to hold the inner box snuggly with maybe a couple inches of padding.