Get hold of a 6V 'flat type' battery often found in 'toy' radio control cars...(TYCO, EZTEC, HITARI etc etc)
1st you need to carefully cut open the plastic case....If you have enough room inside your car/truck, you will be able to skip this step and just solder onto the tabs on the side of the battery.
inside you will find 5 'AA' 1.2V rechargable batteries which have spot welded tabs connecting them in series.
seperate them (but leave connecting tabs in place) into 3 and 2 and VERY carefully fold the 2 on top of the 3 (this is a bit tight, but with care can be done), secure them neatly round with PVC tape.
extend the existing leads as you wish and put a suitable connector on the end for use/charging.
If you open the charger up that comes with it, you can even take out a pair of leads to charge the pack whilst still in the car/truck.
I have one of these 'home brew' hump packs fitted in both my Savvy (with MG995 servos) and the CEN and they work just great......!
(fitting in the Savage requires two plastic tabs cutting off in the front of radio box lid)