You should fix it with fiberglass resin and matte or cloth or both, really.
LOTS of info on YouTube about fiberglassing (esp. boats) and LOTS more here on FGRV in the search engine. Try the "Fear of Fiberglassing" thread on FGRV for a good, long and varied discussion of how a newbie to fiberglassing learned to fix a 1973 trailer's fiberglass from soup to nuts. (from pinholes to a 2- x 3-foot hole, cracks, rough areas, etc.).