is the blue portion the fiberglass portion?that's alot of area needed to glass that would be much better done in mdf. i know you say you need it for the air space but you are going to need TONS of layers along with bracing to reduce flexing. it will happen, believe me. i did an enclosure for my xxx that ran off 1500w and it flexed like a mother, even put a 3" crack in my trunk floor. and it was done with about 15 layers of mixed mat and cloth.
as for attaching the mdf to fiberglass, you don't need any screws. just glass the mdf to the fiberglass. though you need a large number of layers to get a solid hold. glass the inside and outside when attaching to get a good seal.