Buy 4x8 sheets of construction styrofoam, cut to fit interior of floor, it comes in 1/2" abd 3/4" and 1" thicknesses. To get your finished height, glue sheets together with aerosol contact cement, then glue a top piece of 1/8 underlayment plywood; it has a veneer grain/finish. Now you have a lightweight hard surfaced flat floor. If you want to hold it down, use some angle aluminum and self tapping screws screwed into the seat bulkheads. No need for carpet, just stain the underlayment, it's as water resistent as any plywood. The styrofoam is a sound deadener too.