Its always a challenge to come up with a suitable method for mounting shelves and the like to these
fiberglass shells. I completely dislike drilling holes, especially in the top or front. Your method of using the attachment points already existing is one of the best.
I've tried several method that didn't work well at all and have one that works fairly well. I buy black rubber air compressor hose, abrade it heavily on one side with a wire brush mounted on a bench grinder. Then I cut away the ensolite and scrape down to bare
fiberglass.
I spread a thin coat of polyurethane glue (Gorilla Glue or equiv.) on both the
fiberglass and the abraded side of the rubber hose and tape it in place with duct tape. Once the glue dries, I attach thin plywood structures to the hose with truss head sheet metal screws.
It may not be the best, but where there are no existing attachment points it is the best I have come up with, so far.