I also use this one for my Mid runners:
http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html
Shorter and overall smaller than a hardcase. You need to add some protection yourself, but even then it will still be smaller than a regular 6 cell stickpack. Here's an example of a pack installed in a vintage runner, that doesn't see much offroad action, so I didn't cover the full pack with protective material:
And the battery outside the car:
