Battery Shelf Rebuild

I need to rebuild my two failed battery shelves. Each shelf is 30W x 24D and holds 4 batteries, for a total of 360 lbs. I would use 2, 3/4” marine grade sheets of plywood for the shelf. The front of the shelf rests on top of a stringer. The old trays had supports on the sides but this is no longer possible, the walls are compromised. One thought is to mount an L bracket to the rear stringer to support the back of the shelf. There is only room for a small (1.5 inch?) bracket under the shelf. Will drilling holes in the stringer compromise it’s water proofing? Another idea is to build some support frame that would rest on the hull. My concern here is that this amount of weight on the hull may eventually cause cracking. Attached is a cut-away view of the space.