It's hard to take pictures in this little space, but these give you an idea how it was done. I can easily fit a group 27 starting battery in there. These pictures don't show blocks and straps that screw in and hold the battery from sliding around.

The shelf board itself is marine plywood, that has been "painted" with epoxy so that it cannot absorb water.

I used blocks epoxied to the hull and the settee base around the inside at the level I wanted the shelf.

The shelf rests on the Blocks, and is epoxied in place using fiberglass mat to ensure it stays put.

I added a Starting Battery Shelf forward of the 4D bank. This required adding a shelf since that compartment is deep and narrows at the bottom.