This is a S U P E R simple design that will allow you to run many different DC accessories off a simple battery pack.
Step 1: Obtain the parts
1 ea - DC Accessory Jack
1 ea - Battery Holder (x8 AA)
1ea - 9V Accessory Lead
8 ea - AA Batteries
1 ea - Velcro Cable Wrap
1 ea - Car iPod Charger (optional)
Step 2: Make your power connection
I hard-soldered the 9V accessory plug to the DC accessory jack and used a bit of heat shrink to make it look nice. Once you've made a good connection, the jack and cable shouldn't be very much longer than your battery pack.
Step 3: Put it all together...
Take the velcro cable wrap and secure the accessory jack to the battery holder... and there you go! Instant portable DC accessory jack.
This jack with my Maxell 12V charger will charge my iPod from dead to fully charged in just under 2 hours. It is also useful for cell phones, pda, PSP, portable DVD player, and most any mp3 player, cd player, or tape player.