the lipo's that i have found to fit are the Orion 3200 and Thunder Power 3200. AMainHobbies.com used to carry a ProTek 3800 that fits but no longer make's that pack.
I have been racing it for over a month and is a great competitor at my track. I have been consistent in finishing in 4th to 2nd place. but usually get beat by someone running more timing on their motor where i am running with none (or stock timing, Havok 3S w/ss 21.5 motor)
I feel that the Orion pack fits better and has a smaller profile. i believe if there is a 3800 pack that would fit also.