Auto body repair & detailing: Crack in fender 2005 3500 Chevy Duelly, flexible substrate, rear fender

Question
 Some one unknown hit my truck on the driverside rear corner putting a crack in the rear fender approx 13-14" long from the bottom up. As it is a work truck my partner wants to use duct tape and bondo(no way). I've done my share of fiberglass work but don't really think a patch from behind will stick to the plastic(probally shrink and expand at a different rate causing it, the patch to fall off) can you suggest a product that will adhere and stay on the plastic.  The body shop wants a lot of coin to replace the whole panel, not in the budget just trying to stop it from flopping around, maybe duct tape isn't a bad idea. Thanks for your time and sharing your expertice..Todd Bishop East Hampton N.Y. 516 446 3435

Answer
If you are talking about repairing a flexible substrate, forget it.

If you are talking of a metal panel, thats another story.

If it is a flexible substrate, and you don't want to replace it, try using a fiberglass cloth and resin to reinforce the crack, then bondo...but the only real way, is to replace it.