Also bronze screws are available that are harder than brass. Good boat building websites and online catalogs sell materials and have discussion forums on material selection, finishes, sealants and hardware.
There is an exterior plywood called MDO that I used to build my truck dog carrier when I ran sled dogs that has a resin soaked fiber facing on it. It is rugged, holds up well, designed for exterior signs, and has a clean look….the local lumber yard can order it or it migh be available at a chain building g materials store.
It was a nice flat sheet better than pressure treated or thinner underlayment or exterior plywoods that seem to warp and twist.
Here is a link - good description:
https://www.hunker.com/12316947/what-is-mdo-plywoodI designed the box with aluminum angle for the corners and infilled with this MDO ply. It looked great, the box held up after years of rain, sleet and snow, and I applied a marine grade finish.
