I got my fender flares on they look great with one exception

Where we had to cut for the Gas cap. Now I need to make some kind of template with the flare on the truck to create a filler piece and plastic weld or fiber glass it on the inside to fill in the gap

What would you use to make a mold or template? Clay, Foam, Cardboard? I want to leave the fender flare mounted to get the correct shape and pattern, then take it off and assemble the made filler piece? If that makes any sense at all

And has anyone had any luck welding plastic or would I be better off just using fiber glass on the backside?

A few pictures and one showing the gap ...