For my building, I use exterior roof/wall grade A project plywood 3/4". To seal, use about 3 coats of water-based polyurethane and paint or stain to preference. I like using Rust-Oleum latex-based paint. Whatever you use make SURE you leave it in the sun to cure for at least 2-3 days if not a week.

You can build individual cages or a rack for tubs to whatever dimensions you wish. A 4'x8' sheet of the ply runs about $20 at Lowes and you can get quite a few pieces out of one sheet.