Since I adhere to the idea that "simple is good", and I'm cheap, my solution for many years is to use CA hinge material as a backing. After the initial hole has been made, I use about 1/2 of a CA hinge to cover the hole and CA it in place. Let it completely set, then redrill the cowl hole through the CA hinge. This stuff id remarkably tough and resilient. It gives good results on fiberglass or plastic cowls.