Another set I built for a buddy a while ago :
EDIT : The flames are actually soft squishy foam, partially for Decor, and partially to help reduce noise on the box by acting as dampers - this was my first semi-serious attempt at a system, and worked out pretty damn well - Sounds GREAT.
http://img.techpowerup.org/100409/DS1.jpg
http://img.techpowerup.org/100409/DS2.jpg
http://img.techpowerup.org/100409/DSPU.jpg
Also powered by a Car Battery, using a little power supply I make myself, a 1A 12V (True 1A - non stop, not 1A Peak) charger basically, with the battery "In-Line" with the amp.
During normal daily usage at volumes the rest of the house can actually tolerate, it runs off the power supply itself, and when you call for extra BOOM, then it uses the battery.
YES, it wont be able to run forever, but if I tried to make enough noise to kill the battery, I'd probably get arrested for public disturbance, and the Speaker set pictured above has been used for house parties before and managed to run all night at party suitable volumes with no hassle.
This Power Supply pictured has been running 24/7 for almost 8 years now (As well as that battery) and still works just fine.
I DO think the battery could use replacing by now, but it still works - so why bother.