see https://www.greatgrady.com/threads/deck-hatch-re-core-265.32638/
As for Core materials:
Plywood can be Hardwood (oak, maple,birch, Douglas Fir) or Soft(pine)
Plywood can be rated Exterior, Exposure 1, Exposure2, and Interior. For core material you can only use EXTERIOR.
The ply can be...