fiberglass sub box?

My buddy is doing a custom box in his civic.  He is using the bottom of the spare tire well as the bottom of the box.  He braced the sides with L brackets and sealed all along the bottom of the wood with monofoam.  Does he have to fiberglass the bottom still to the inside of the box to seal it or will the monofoam work?  He said someone told him he should fiberglass it but he wants to avoid it if he can.

Thanks