I have the same setup, but only needed 1 oz of weight in the tail to balance.

I did not move my battery back, but you can install a battery tray just behind the hatch. You can cut a piece of plywood to just sit on top of the ridge on each fuselage side.

BTW, I found the stock engine mount too flexible for the Satio 91 and went with a Dave Brown 90FS engine mount instead.