JB weld is more of a metal patching aid. Not sure if it works on plastics.
I have a crack on the front of my girls 250ex that I did some work on. Here's how....
First I made some semi-deep scratches a touch before the beginning of the crack on both sides of the fender (top & bottom) to stop the crack from continuing to travel any further. I made the direction of my scratches go in a perpendicular direction to the existing crack. For example, you're basically making the top line of this "T", but leave a little space between your current crack and these new scratches.
Next I used a marine adhesive made by 3M that's called "5200" to strengthen up the underside of the crack and provide stability. 5200 is virtually impossible to remove once it's fully dried, which can take 1 week if you don't get the fast cure stuff. I used a little sandpaper to scuff the area I wanted to apply the 5200 to and then cleaned it with rubbing alcohol prior to application. Once the 5200 is out of the tube and on the fender, you can use a wet finger to move it around and/or smooth out the finish.
Because it sounds like you need more fender support than I did, my suggestion to you would be:
1. do the scratches mentioned above
2. then do the wire-tie weld BB mentioned
3. then do the 5200 reinforcement
Let us know which way you eventually end up going and how it holds up over a little time.
BTW, the crack in our 250ex hasn't been a problem since my fix 4 months ago and we've trailered and ridden at least a dozen times since then.
im just kidding lmao
