Unfortunately, when Meatballman and I built the one earlier mentioned we didn't take pictures or I'd share them. Just a few thoughts on the complexities at hand...
1. Battery life... a motorcycle battery will last about 3-4 hours, so anything smaller would be less. This was the major contributer to size, besides the sub.
2. Battery recharging... the battery charger is on board with ours so you could cut that for size issues, but then you've got to be able to get to the posts to recharge which we didn't.
3. The speakers will of course need to be amped, which is the reason we chose computer speakers, if you're running normal speakers with a cross you'll need an amp which adds more size and probably more power than the computer speakers were pulling.
4. Weight... all that stuff is heavy... that's why we chose the rolling option....

Your concept is cool, but unless your talking 1980's Fat Boys style boom boxes I think it will be hard to fit everything you need inside, and even if you could I wouldn't trust the handle.