Plywood!! Cheaper, lighter, easier to work with.

My rack shelves are 3/4" thick exterior grade ply, covered on the tub side with plastic sheeting. I did put a coat of poly on all the wood before building, just for extra protection. And the paint I used was Latex based Rust-Oleum. Just make sure you let it cure outside for about 3 days and it's good to go.

I'll be using melamine shelves for my hatchling rack, and I already have the wheel casters for it just to be able to move the thing. I wouldn't be using melamine at all, but I had bought/cut it for another rack plan that didn't work out, so now it'll be a hatchling rack.