You can buy premade sheets of FRP laminate from McMaster-Carr. You would still have to fair and prime them, then paint them, but like he says, they will NOT rot. Or you could use Coosa board. But then you have to glass over it.
Or (still thinking about this) you could epoxy a sheet of 1/8" fiberglass panel onto Coosa board and then cut it to fit. It would still be light and it would not rot ever.