I'd seal the surface to be glued with straight epoxy and before it's totally set use thickened epoxy to attach to cabin side. Best to dry fit it first and plan how to hold it there until the epoxy sets - overnite. If you're lucky there will be a few places where you can use screws into something other than the fibreglass cabin side. Where you have to apply pressure to get it to stay cut scraps of wood long enough to brace it to something else in the cabin. Good luck.