I may be able to use both your ideas.....2 batteries right behind the servo tray is just about the right amount of weight.But I'll have to cut a hole in the fuse to get them back there and make some sort of brace to keep them from sliding around.So perhaps cutting a hatch in the bottom of the fuse is the way to go).

When you use a Y harness with 2 batteries,aren't you doubling the voltage in the process?I saw another idea where you simply put one battery into the battery slot,and another into an empty servo slot.