I see dudes carrying guitar bags on their backs all the time. I assume they've got two straps like regular backpacks. I once saw a band called Blind Pilot that was touring the west coast by bicycle. They used trailers on their bikes. The stand up bass had a case that looked like a coffin being pulled behind the bike.

I found a pic of it.. heh..
http://www.flickr.com/photos/blindpi...7606763393604/