I have been worrying about this aspect for a few months now, and this is what I intend to do. Fibreglass tap along the seam, maybe along the bottom too, it might not look great, but it should be pretty hidden too.
The marine ply boards are 9 and 18 mills thick respectively will be fiberglassed with matt to the bulkhead and the flat. The aluminium bracket will be screwed with maybe six/eight or more wood screws to the bulkhead and flat, the idea being that the load will be supported by the bulkhead as much as anything. I had thought about putting the lead crystal battery on the frame of the trailer, but then it is pretty inaccessible.
I hope my photo came out…
