You should be able to balance that plane without adding any weight. That battery pack can be moved all the way back till the fuse is too narrow to accept it. Just move it back along the fuse till it ballances, cut a hatch, snake the cable & any extention required, install and replace the hatch. Heck, I had to surface-mount my elevator & rudder servos way aft on my .40-size plane when I mounted a .61 in it.