Just use. 1\8" plywood. Cost both sides with a mold resistant primer and the interior side with a good quality bathroom acrylic semigloss enamel paint. That is what I did and it has held up great going on year four full time. No problems with mold and it is easy to clean. Plus it weighs and cost a lot less than FRP.With insulation filling the void between “studs” and with the studs also running lengthwise I didn’t think it would be too much of issue. Now if that doesn’t provide enough structural support I could always trim a 1/4 plywood and then adhere the FRP to that; correct?