Boy I am slow at posting,3 while struggling with my post typing is a one finger operation for me.
Brunog yes ridig urethane panels can be used in E/G.You were actually refering to polystyrene foam which does melt i n polyester.
The density of polystyrene and strength is of little use in E/G,urethane is the way to go.If you laid down 1"ofE/G and put in 6"squares of P/U foam separated 1/2" and poured over this would form ribs connecting the top and bottom forming a lite stiff pannel.
Larry