question: Polystyrene & fiberglass

You would be better off looking into Plascore or something similar.

 

If you build a closed space and don't want the foam, you can dissolve it out after you are done.

 

You can use Polystyrene foam as a plug or former if you paint it thickly with PVA to protect it from the polyester resin. It is not suitable as a core material. If you want to leave the foam in place after the outer skin is complete, cut away the foam in the form of ribs and stringers before laminating the inner skin. This will create a sort of I beam structure that does not rely on the foam for strength.