We have come to a final design for the battery box. It can accomodate up to six size 4D batteries or fourteen golf cart batteries. The overall height is about 15 inches which means it can handle the taller golf cart batteries (like the Trojan T-145).
Now T-145's weigh about 80 lbs each. In order to ensure the weight will be supported, extra and beefier crossmembers, between the lower tank and the box, will be installed. The box itself will be thru-bolted to the framing supports all around it. Cleats attached inside the box will keep the batteries from sliding, and there will be five hold downs bolted across the top. Attaching different size blocks as "shims" will hold down the different height batteries. The inside of the box will be glassed with reinforcement in the corners.
Cutting down the center tank to make room for this box reduces the tank to 37 gallons. With the wing tanks at 69 gallons each, that still leaves a total of 176 gallons for fresh water. With a water maker on board, I think this should be ok. Heck, we seem to get by with 20-30 gallons on the small boat.