If you find an 8.4, 2000mah battery, it will work. Even a 1700mah will work. No need to go overboard.
When you reach that much MAH capacity, sometimes the cells of the battery pack a a bit larger, so they dont fit well inside the stock. 9.6 volts wont fit at all.
He means he had to force his battery in there.
Before you go on a spending spree, go through the age-verification process. And there are two gaming sites in the region you can visit.