I just mounted a large ( 3800mAh NiMH ) battery pack onto a ply tray using velcro straps, then glued the tray as far back as possible behind the wing in the fuselage.

This brought the C.G. to the 4" mark.

The further back you can get the C.G. ( even 5" is good! ) the more aerobatic the Stik becomes.

Mine hovers and torque rolls with ease, much to the surprise of some.