The stock can use any normal battery with a tamiya connector. However, because of the tamiya's size, you'll find installing the battery less than ideal.
The gap between the 2nd and 4th cell is required to lock the stock when it's folded. This gap is for the brass deflector to protrude through. If you don't intend to fold the stock, then you can use virtually any normal "rectangular" shaped battery.
AEG upgrades & repairs: $25/hr