OK, I know there are some Optima Haters on here, but I just upgraded to run 3 D31Ms as my main batteries on my boat. I got a custom box/tray made to hold them. The tray locks the batteries at the bottom. The box part of the enclosure also holds the batteries down and helps strengthen the over all box. Since I have a Centurion Enzo 216, I had the top piece engraved with the Centurion Logo. Attached are pictures.
Also, I have the 3 batteries separated by individual two position switches so I can operate on a single battery if needed. I have a Guest 3 bank charger that charges each of the batteries. I also have a 4th battery that is normally switched off that I can run the boat from in case the 3 bank system discharges too much during a day of partying.