10 years ago I made a mould off of my Manx dune buggies rt rear fender for a friend. A number of coats of a release wax, then glass and resin. It popped off great, no damage to my fender and the replacement piece was made from it. No gel coat or color but it all was painted later. Think it came out to be around $75. If you could talk some one into using their box for a mould......
Actually you wouldn't need to make a mould. If you're careful with the glass layup your finished product would only be a tad larger than the original, that may work out fine. There's my 2 cents worth