I would suggest remove the battery and weigh it, then enclose the battery in a submersible streamlined container and suspend it below the hull, make the container and connection to the hull as light as possible, this wont then add more weight to the downward force on the hull, the deeper you suspend the weight, the better the righting moment will be, the issue is connecting the battery back into the boat and ensuring water tightness.
Drawknife - could you post a picture of the boat, the battery and any heavy item that could be transferred to a submersible container external to the boat and then connected by extended cables. Also how big a boat is it, as it has a lead bulb and fin, just moving the battery to under the vessel will significantly improve the situation for the same weight. So changing the fin to have a large bulb which contains the battery and the lead weight below that will improve the situation.
If you look at the picture above you will see the fin and bulb (which is removable for transporting), the original was virtually twice as deep and was a brass tube with a square plate flat soldered to the bottom, it had 3 x 53g square washers on top of that plate, you can just see it under the hull in this picture. The righting moment of this was really good.