You can take two or three stock "minibike" fenders, weld them end to end so you have one long fender, then cut some 18 gauge sheet metal to the desired design and weld those pieces to both sides of the fender, this will give the fender sides...From there you can can tack small metal tube around the edges and mold them in for more of a bobber look... This isn't as nice as one that is totally hand formed on an English wheel and such but it can be done for a distinct look and at a reasonable cost..