I know that friz mounted a sealed AGM battery under the cabin. The battery he used looks like it has a rugged case. But then I’d need to decide where to drill holes to get power from the charger to the battery and from the battery to the distribution panel.
In retrospect, you could glass the outside corner between the forward edge of the flat and the forward face of the bulkhead, before the shelf is installed. I don’t want to do that now. What I will do is re-glue (thickened epoxy) the joint then screw a 1×1 piece of hardwood onto the bottom edge of the flat into tie bulkhead. I’ll use #6x 2″ screws and work hard to make sure the pilot hole is in the center of the 3/8 bulkhead plywood, and dip the screws in epoxy before screwing them in. The bulkhead plywood won’t hold the screws real well, but if I use enough screws it should hold. I will also put some kind of floor in the galley module which should help to spread the force of the vibration from the 60# battery. As a belt and suspenders approach I will use a scissor jack in the cabin to support the joint during transport.
it’s possible that the joint failed because I wasn’t able to properly weight the galley flat when I initially glued it to the bulkhead (when it was still upside down).