I have my E5 on BEC, mounted to the back of the ESC tray, and my E5s on Lipo. The Lipo is strapped to the top of the front pack like Chumley suggested. Helps with CoG on lighter packs. Keeps the packs farther into the frame for crash protection.
Putting in the back of the battery tray is also a popular option as Knuckledragger stated.