We placed the new batteries to the aft cabin last year, but the shelf we made was not constructed very well. It was done in a hurry and was not thought through well either. It was supported by legs on its port side and the hull on the other, which was not keeping up with the weight of the batteries, so the legs were bending and the shelf was sliding towards the steering gear.
Before sailing out this year, we decided to redo it.
The old shelf was removed and used as a template. We bought some fresh wood and a thicker plywood sheet and coated everything with epoxy to make it water proof.
The wooden planks got carefully leveled and screwed into the bulkhead on one side and epoxied to the hull on the other side with thickened epoxy. The plywood shelf is screwed into the planks and the batteries are secured with pine trim pieces against side movements and also by several straps. It's a much stronger construction now, should be rock solid.
The shelf also got painted with two coats of Danboline to look nicer.