I have a 1988 K car wagon that im replacing the heater core in. One of the mounting posts and some surrounding plastic was ripped from the back wall of the box. I tried a ABS type glue to tack in in to double up with a fiberglass patch w/ resin. But to no avail. Anyone know what resin or epoxy to use? Im of the opinion to use polyester resin as it stays flexible where Epoxy gets hard and brittle.