I am installing a couple of shelves in a cabinet that will have to span 38 inches. They will be 20 inches wide. I would like to use 3/4 inch plywood but I am not sure that the shelf will not sag. I am thinking about making them out of oak but It would increase the cost quite a bit and I am not sure the oak would not sag. The cabinet is furniture grade so I just can't support them with 2×4's etc. They will not be loaded with a lot of weight, just stuff like shoes,sweaters maybe a jewlery box etc. What say you?