Here is my setup, it can handle really long lipo's and it uses the exisiting motor mount holes so you don't need to drill holes in the chassis. It is basically a flat plate with a small aluminum bar stock bolted to it, and this piece bolts to the chassis. the battery is held with a combination of velcro and a strap. The plate is very rigid and keeps the battery from hitting the spur.
Just make the plate as long as you need for the length of the battery your using.
Flat aluminum plate bolted to a small aluminum bar.
Updated version using carbon fiber instead of aluminum.