from what i'm reading , i think the easiest/fastest/cheapest way to do it would be
1) take the current fender and using hot glue, wood, masking tape turn it into a 'positive' mold
2) using a hand-lay fiberglass make a 'negative' mold on the outer surface
3) lay the new part up inside the 'negative' mold
if you do it that way it wont be as nice as doing a pressure set 2 part mold "the right way" but you'll still get a reasonable quality copy at 1/10 the work of doing a full 2 part mold with a wooden buck. you might be able to get creative with spray foam and blocking and masking tape and make a hokey inner mold just good enough to put some pressure on there. i've done some raggety-ass shit by literally just stuffing rags wrapped in masking tape into cavities to put pressure on the inside of a mold. i was also thinking the other day about maybe using baloons for something like this? like clown ballons wrapped in rags and taped into place?
i dunno fiberglass is pretty easy, if you can do paper mache, you can do fiberglass. i've made some janky shit come out pretty ok looking.