Cut away all the marginal stuff 'round the edges, try to end up with square edges. Make up a template with card that matches up nice and tightly, and cut a patch to match - I'd do a rectangle, and take thin V shapes out of the area that will end up going on the underside of the wheelarch, so that it can curve smoothly. Bend approximately into shape, tack the outside bit first and then tap the underside into shape gently - once you get the nack of doing curves it's not so hard 
