Originally Posted by
Steve Weiss
Exactly what Jason said: Tungsten is dense metal commonly used as ballast.
The tungsten I have comes in small plates and I have servo taped them strategically to the lipo pack and also used battery strapping tape to add a second level of security to keeping them where I want them.
I will try to take some pictures later of my packs to give you an idea of what I have been doing.
Yep, copy and pasted right from
http://www.maximum-velocity.com/tungstenplates.htm 
So i wonder if it would work to crack open the plastic case of a lipo, and add tungsten plates to the inside? That would keep a nice clean look...