Same concerns. Plywood will likely sag. Thanks for sharing your thoughts on the question.
I found
a calculator for shelf sagging. And it states that 12psi is too much for plywood. This is for 180x210mm shelf size - close to that of a 304Ah cell. If the parameters are entered correctly and the load is uniform on the surface the 18mm (3/4") plywood would sag close to 1mm.
So it seems that an 8mm aluminum plate will be used inside my enclosure. And this brings the complication of insulating it properly to avoid the chance of a short circuit.