To go ultra-light weight, I considered buying a 55-gallon blue plastic drum. Cut drum in half vertically. Use the bottom as an attach point on the trailer. Go up the side of the drum as far as needed to get the fender outside the edge of the tire, and cut drum off there. Done right, one drum = one pair of lightweight, waterproof, non-rusting fenders. They could bolt to the trailer side with three bolts.