My first thought was to take the tool tray out, line the top of the mudguard with a couple of sheets of clingfilm folded up over the frame sides and then lay on a couple of layers of fibreglass, wetted down with resin, should lift straight out afterwards and trim with a dremel. Still considering this option.
What I'm thinking of now is using a thin sheet of PET. Get a 2 litre bottle of your favourite soft drink, cut the top and bottom off, slice down it to make a sheet and then apply heat (hot air gun?) to heat it enough to mould. I know you can do this to make moulds of things for models but not sure how this will work in practice on this scale. I only have small, fine point heat gun, not a big hairdryer size unit.