It was probably plywood. Use 5/8 or 3/4 and if it flexes too much put
some pieces of 2x2 attched to the underside of it. Cut to fit the
opening. Of course you want to put a piece of foam on top and cover it
with vinyl.

Mike wrote:
I have this 1995 Baja Islander 208 and it is completely missing the
sundeck and I am needing to know the dimensions to rebuild and what it
is made of and anything else you think I should know or need to
complete this project.