I have rebuilt two sets now, from road damage. Here is my trick.
I use 1/4 inch Masonite panels cut to fit the flat panel areas and then use Cleco sheet holders [used in building aircraft and race cars, look them up] to hold the Masonite panels in place as backer for the fiberglass mat layup.
I coat the Masonite [smooth side only] with mold wax so it releases from the fiberglass once it sets up, Cleco the parts in place after I feather the edges and rough them up to take fiberglass layup. THEN, lay up a layer. Let it cure, rough it up again, put down another layer, repeat..
I've found it works best if I can use the Masonite sheet on the outside of the fender. It leaves a smooth, flat, surface that doesn't take so much grind and prep to get the "new fender" look.
How you manage the layups and putting the pieces into place to make the fender will vary, depending on what pieces you have left to work with.
Bottom line, it will cost you a lot less than a new fender, in mat glass and resin. Rule of thumb is, 4 layers of well saturated mat, and a clean surface to glue your parts and mat to... Remove ALL oil and road grime or it won't bond.
Good luck. [take the fender off the truck, so you can always work "flat" instead of overhead.] I made a wood buck to mount the fender to that mounts to an engine stand so I can rotate it. Well worth the effort.