I am going to be bulding a rack here in a couple of months once I am home from Afghanistan and while I was intially going to go with PVC I have decided to switch to plywood. I will be using a sealant on the bottoms of the shelves to prevent the humidity within the tub from damaging the wood. Aside from weight are there any real drawbacks to using plywood as opposed to expanded sheets of PVC (aside from the fact that PVC will last just about forever)?