http://www.hobbyking.com/hobbyking/s...po%5FPack.html
I run one of these in my SC10. It's a bit longer than a stock one but there is room in a b4 I think, I can check in a bit.
A soft case battery buried inside the chassis of a car under a large battery strap is pretty safe in my opinion.
I got nearly 40mins with a 10.5t Novak motor bashing around the grass dunes at Gwithian.