These now seem be be produced by Skillard and available for pretty cheap.
Skillard Front Bumper Delete plates:
https://skillard.com/products/280z-front-bumper-delete-plates
> These are custom made front bumper delete panels that are sold as a set. They will fit the 280Z, 260Z and 240Z. They will work with aftermarket air dams as shown or with the factory lower valance. NOTE: this modification is not DOT approved.
> They are CNC cut out of .050" 5052 aluminum and have M5 stud inserts that allow for easy installation. These are intended for off-road use to clean up the look of your front end when the bumper is removed. Mounting hardware is included.