It's unsprung weight and small, so weight is not that important. You need to build for impact resistance, so on a bike that will hit the dirt often forget about Carbon Fiber. You'll need a 3-3.5mm laminate due to the vibration. So I would go with pure fiberglass bi-directional with epoxy resin. It's very drapeable and wets out nicely. About 3 layers of an 18oz would be sufficient.