I don't have any advice for flying with a box, because I have never attempted to fly with an accordion myself. Here's a thread where it it discussed...

http://www.mudcat.org/thread.cfm?threadid=37968


There are several Mudcatters who fly with their accordions a lot but most don't seem to be here that often (Sam Pirt, Skipjack, Bill Sables, being a few of them). Maybe a PM to some of them would help. Good luck!