External isn't too hard, and you wont risk messing up the car. You'll want to get the extra batt to sit above the back wheels (I think) for balance.
I have a diecast ambulance that's top heavy. Most of the weight is in the back. I use oil coated slick tires. I loose acceleration, but it never flips. Besides, it's a diecast ambulance. I don't really care about acceleration with that. It might beat a stock. It is dual celled (internal). Although, I could always just try and kill the competition. It could probably knock a regular bit way off the track with a good hit. It's like a hot-wheels car frame that I dremmeled out to fit a bit. It's pretty heavy. Anyways, happy modding.
Also, if you are worried about damaging the inside, here's what I do. I disconnect all the internal wires, except the steering wires, from the chassis. Most of the wires, with the metal contacts, can be pulled out one way or another. Unscrew the steering comartment. Take all the pieces out of the steering compartment. Now move the PCB to the front of the car and secure it somewhere at the front of the car with a piece of tape or something to keep it from moving. The idea is to get the batt compartment free of all wires, so you don't risk damaging them. Now you should have a good clear space to dremel/cut away. If you cut too much, there's always duct tape and superglue. That fixes anything.
