Shopping for new rack shelf material.

[Godzilla78]

So I am renovating my current rack, I bought it homemade from a guy off craigslist as package deal.
(it came with 6 snakes!). The heat tape setup, and the tubs are perfectly fine, but the rack shelfs are made of fiberboard, which is not humidity resistant, and they are warping and they are basically crap! So my plan is to save the current tubs and heat tape, but remove the old fiber-board rack shelves, and replace them with more water-resistant higher quality rack shelves. I looked into buying sheets of PVC, as it is very strong and totally water-proof, but the sheets are a $fortune!!!, especially when you consider shipping! I might as well just throw the whole rack in the garbage and buy a new Commercial made rack for the price of PVC shelf panels! Plywood seems to be the best choice, as it is more economical and I can get it anywhere. WHAT KIND OF PLYWOOD is humidity resistant? there are so many kinds! I plan on getting ¾" thick sheets to prevent warping under the tub weights, and I want make sure to get something that is resistant to humidity and water, and won't warp like the current el-cheap-o fiber board that the last guy used.

Any suggestions on good rack-shelf materials? I was going to go with a high-grade plywood, and then maybe laminate it with an acrylic paint, or other sealant.