Sheet plastic is the best material for racks and cages alike...

However, if you're poor (like me) or just can't find a supplier easily, then the better material is 3/4" plywood sealed WELL. I made a cage system of basic exterior roofing ply and it's working great so far.
Of course, I take mondo precautions for waterproofing and not only did I use water based polyurethane to seal the wood, but I also painted it with primer and two coats of a latex-based paint and covered the inside with contact paper. And made the back out of Corrugated Plastic sheeting.