wet core

looking for recommendations on repair material. i have removed the tabernacle fiberglass skin and wet core from my c&c27mkiii and found what was left of a piece of marine ply that was sandwiched between the skins under the mast step. do i use another piece of marine ply? stainless plate? or...

I have researched this topic multiple times, so I felt comfortable enough to cut into the deck of my O'day 22 to begin replacing a wet core beneath the mast. I first cut out about a 12" x 12" area immediately surrounding the mast. I expected the core to be wet and weak, but this core was POWDER...