Mud-flaps
Fitted ...

... but a bit clunky, sticking out past the wing, so I trim about 1cm off the curved edge of the off-side flap (left), which brings the top corner inside the wing.

No metal insert so the screw tightens down until the head bottoms on the clamp, cutting into the rubber and limiting the clamping force onto the wing flange (top clamp). A thick washer with a small hole spreads the force over a larger area, creating more clamping pressure (lower clamp).