I've traveled with an accordion before... It got completely trashed. I'm doing it again soon though, and I've got a better plan this time. I use a 48-bass accordion with 4 columns instead of 6. 48-bass accordions are way lighter than the 120-bass versions, and the 4 column version gives you a fuller range than the 6 column. All I use for bass buttons are major and minor chords so I don't need the extra 2 columns.
The first accordion trip I made was last summer traveling around the north east. I have this pack:
I would pack my bag as lightly as possible and pack it flat. The accordion fits beneath the strap in the middle, and when tightened, the strap alone surprisingly kept it firmly in place. Only problem with that... the strap crushed the already worn to shit grill on the treble side! When it was raining, I'd wrap two trash bags around the accordion. My whole pack, accordion included was able to fit under the cat-walk part of a mini-well. After traveling with it for a little while, all of a sudden I'd hear a something clunking around inside the accordion... whoops, looks like one of the reed plates just fell out. This happened more and more and now my accordion just sounds like a pathetic ghost, which is kind of cool but not the sound I'm going for.
Accordions don't do well with extreme temperatures. Being constantly outside kills an accordion. I think busking a bunch in the fall last year also contributed to the death of that accordion. Cold fucks up the wax that holds the reeds in place causing it to crack and reeds to fall out. You're supposed to wait for your accordion to warm up for an hour if you go into a warm room before playing.
this site has more information on that.
I'm really interested in getting one of these hohner 48-bass backpack cases and sewing a bunch of side bags onto it.
I want to find a good material to waterproof it with and use some of that seam sealant stuff that REI likes to give away.
Traveling with an accordion is totally worth it for me. I've only ever paid $100 for an accordion, and I definitely make way more than $100 from busking. It's also sweet to have your instrument with you all the time.