Sail panel backing boards, what are they made of?

I just installed some in my 67 cuda. Like $20 from classic. Made of 1/8" hard board.

Yep. just like the stuff they put on the back of cheap furniture.:D
I made the rear deck cover of the same stuff, then covered it with 1/4 foam and black speaker face material.
6x9 speakers still mount in the openings from the bottom side and you can't even tell they are there.