The problem with using plastic rather than wood is that the acid that leaks out of the batteries will run off the plastic and onto the aluminum bus bay. Plywood will absorb the acid, and keep it in place. Yes, over time, it will deteriorate, but when it does in 5-10 years, you can just cut a new piece and replace it. It works very well and keeps the acid from eating your bays.
Definetely plywood over plastic for batteries unless the plastic is in the form of a fully contained box.
I am using AGM batteries so I have bigger problems if they ever discharge any acid. My golf cart batteries seemed to always be coated in a layer of crap even though they never seemed to need any water when I checked them.