Well, unless you are willing to pay otherwise the PEQ II can only store the small battery. The custom Elite cell for PEQ II can store up to 9.6 V 4200 mah. It would be enough at least for 3 or 4 games or you want to change the stock to crane stock, STAR Crane stock or D-boy crane stock can store up to 9.6 V 3600 mah as well. Both option are bigger than your 2800 mah. Hope it help.
Edit : you need to change the battery as well if you want one of those option that I mention above.