This is to go at one side in our high-ceiling enclosed front porch, no fire load apart from the aluminium ladders which have lived there up to now.
It will hang off the floor joists of the room above (bolts or coach bolts through or into the BIG timbers) so the joints will all be in tension, which is why I fancy a simple fully-bolted framework.
I have seen reference to batteries on shelves in racks, but why? Would it not be possible to just bolt Dexion-type angle to the side droppers and sit the batteries on the ledges each side? Mock-up, having raided the grand-kids inheritance (not to scale):

I would clad the non-wall side and put a door of some sort on the front, probably a ventilation mesh underneath and across the front top above the door too.
The Pylontech leaflet shows them stacked using their bespoke brackets (max 4,) or somehow using (just?) the front rack-type mountings. A skeletal rack with no shelves seems simpler...
A