RE: 27% Cessna 421 Twin Turbo Prop
That is simply dense foam insulation. Much easier to shape than to sheet this area with balsa. Once final sanding is complete, the foam will be covered with glass cloth and West Systems epoxy resin.
The build has slowed, as I am waiting for the new Wren 44 Turboprops to arrive, which will be sometime later this summer.
cheers,