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You can also use body bond (aircraft glue) to glue screw retainers to the underside of the car. They will not come back off without damaging the paint and possibly denting the metal, though.

Tee nuts facing the wrong way, glue the flat side to the body of the car, drill the coroplast to go over the sleeve of the tee nut, add a screw and washer to hold it in place. May be a good idea to glue some butyl gasket material to the side going against the car to keep it from vibrating.