I heated up and bent a 90 degree into a piece of coroplast and screwed it onto the underside of that black plastic piece. The coroplast is flexible enough to bend out of the way when contacting the road surface (easy enough to do when carrying a heavy load in the back) and can easily be replaced if/when its gets eroded down too far by occasional scrubbing with the road's surface. The back portion of the side skirt especially tends to scrub the road when going over high curbs, etc. When I installed my underpan, I slipped it between that black plastic piece and the car's body to hold it in place before drilling and tapping screw holes through to the metal body.
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