If your worried about strength, forget the bedliner... grab some fiberglass cloth or csm (chopped strand mat), and some polyester resin, if your doing this on the inside of the fenders, you dont have to worry about being sloppy, or air bubbles, as the inside finish is allready rough.
There are many great videos on youtube about fiberglassing tecchniques... But basically scuff the interior with like brillo pads, trim a couple peices of cloth/mat to fit, brush on the mixed resin, lay the cloth, push out any bubbles and stiple it so it sits tight against the old fg. Easy intro to composites...