1992 V20 - Looking to add a second second battery

My '91 has the battery switch and one battery on the starboard side and the other battery on the port side under the rear hatches. You had to remove the liner to get to the switch so I installed a new hatch near the floor for easier access. I removed the oil tank from the port side because I went to a 150 Honda 4-stroke and installed a wash-down pump. I now want to install a set of Bennet trim-tabs and dont' know where to install that oil tank.