I just came from Home Depot (Because that's all we have in town)where I was looking over their (quite extensive) collection of blind hole fasteners. There is something out there I suspect; at all of the various hardware stores. The trick with a ladder is to get good backup so the weight of a person does not cause the fasteners to pull out or crack the adjoining fiberglass. And of course on those holes where you can access from the inside by all means use as big and as much backup as you can. In some cases you may have to form a backup from hardwood. Oak works good for that.