Do you think it is the flexing of the insert that is causing the cracks? Or is there too much pressure on that curve in the insert? Either of those causes is going mean that your idea of an aluminum strip embedded in fiberglass on the bottom of the insert would help to prevent the cracks from returning. The expanding of the crack and refilling it with resin and glass fibers would also help to hold everything together. As far as LineX on the insert, I have not decided whether I want to that or not.