I'm kinda late here....I have been using Fusor products. They have a wide range of 2 part panel adhesives, they supply you with a type caulk gun that has twin tubes and a mixing tube, Some cure in 1 min, others with heat and some just take longer, some are flexible and some get very hard, If you are going to "mold" them in, your going to have to finish the surface with polyester putty. You can be assured that the Fusor will not let go. I flared some fenders on a MGB years ago. I made the flair on the car with fiberglass and bondo.... only 1 cracked after 10 years and that was because the tire went into the flair under a hard leave on the right rear. I would not worry about different expantion rates, or use a flex agent.
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