[straightshooter]

Plywood would do just fine with that small of a span. I wouldn't use less than 1/2". Besides plywood will take up less thickness so you can have more shelve space.
My ammo cabinet is all 3/4 plywood and shelves loosely sitting on wood supports screwed to the cabinet sides and back. I mean to tell you I have a lot of weight on them shelves and no sagging yet.