Chris,
There is a new type of 'speaker' out that is exceptionally portable. It's basically a cylinder about (according to my faulty memory) 50mm x 35mm that vibrates. You place it on a hard object and the cylinder transmits the vibrations (sound) to the object which then acts as the speaker. I know it sounds weird and unlikely to work but apparently it does and sounds reasonably good. A mate, who used to work as a sound engeneer and so knows about sound quality, heard on sitting on a styrofoam crate and was impressed, given what it was. I heard a demo on a cardboard box and thought it wasn't too bad but that was spoken voice, not music.
I've forgotten the name at the moment but they have them in the bookshop at work, so I could check up on Monday. You might then find some reviews and decide whether it is for you.
EDIT: Here's something similar
http://www.techradar.com/au/reviews/...-696679/review