widening fiberglass dually fenders

I am in the process of widening my fenders to clear the 19's with a 3" body drop on my chevy c3500 dually. I spaced the fenders out 1.5" on each side and have the bolted back on. Now I am left with a 1.5 gap between the steel bed side and fiberglass dually fender. How would you guys suggest filling it in. I don't want to mold the fiberglass to the steel either. What I was thinking was to tape off the steel side and put fiberglass matting in the gap from the bottom side and just start building it up with f/g resin until it is close to shape and the do fiberglass filler and body filler to finish it off. Will it adhere to the fiberglass fender or will it crack eventually? I was also thinking of not molding it to the fender either and just keeping it a separate piece in between the bedside and the fender. Any suggestions? Please help!!! Sorry for the novel but I need to get this done soon.

yeah it would be better to cut the fender in half long ways and strap it together and put some duct tape on the back side of it then fiberglass the gap, then sand then paint...

I already have the mounting flanges made. I used 1.5" c channel and bolted them to the studs in the fiberglass then also bolted that to the bed side. so the mounting is very sturdy. I thought about doing what you suggested but with the way the fenders are angled as they get wider the bed side piece would have to get wider also or other wise the filler piece would have to be at a 90 Degree angle to get the 2 to line up. I know what I am saying is hard to understand especially not looking at them. Thanks for your help

I would use fusor 100EZ it is a two part epoxy to me it is stronger then fiberglass and it cures in 15 mins with heat, you use fiberglass cloth and then put the fusor on it spread it out and heat it up for 15 mins and shape away.