If you're not going to play keys, I suggest a laptop and a sound module (SC88, MU50, X5DR, JV1010). Band in a Box is not for live playing; you can make an arrangement in advance and play it, but you can't change chords and styles on the fly.
The other possibility is a MidJay from Ketron. If you want something real compact you could check out a Yamaha QY100. Lastly, what about a Boss Looping machine, RC20,50 ?