After putting a hole in the back wall of my Eagle on the first ever outing, I just picked up some epoxy putty at Home Depot's paint dept. The one I got was for use on plastic/fiberglass, and dries white. I never bothered painting the patch, and it's barely visible. Still water tight after 1-1/2 years. Should do the trick nicely!
2013 Eagle 266RKS
2011 Ford F-150 w/3.5L Ecoboost & H.D. Tow Package