The cabin top construction: a layer of fiberglass, plywood or balsa, and another layer of fiberglass is all part of the structure that keeps the cabin top stiff and strong. I wouldn't (didn't) skip the interior layer of fiberglass.
I'm sure my prolems started with holes drilled ito the cabi top and then thigs screwed directly into the plywood. proper way is to drill out a biger hole, fill it with epoxy, and then screw into the epoxy. A little more work up front but saves $$$$ and times later.