advice please

[Wombii]

My fenders weren't the same style, but the same idea should apply. I would use all 1mm styrene plates and layer them if necessary.
Cut a rectangle and bend it to the right curve.
Cut out the side profiles and glue one on each side of the curved rectangle.
The side profiles will help keep the curve in the right shape.
You may want 2-3 layers for the side profiles both for stiffness and to get it thick enough to sand the edges.