So what do you do with these after they are fabricated? Got any extras?

Why not do these in 2 setups, start with back and machine threaded mounting holes and cavitate the back the back using say a mastercam solid shell to reduce weight and machine a stand off area around the mount holes then bolt it to fixture plate and machine the tops complete to eliminate the hand removal of the base slab and the threaded holes would make mounting them to the bumpers easier. of course if these are vacuum form plugs disregard this. what would probably be cheapest to mass produce them would be to do a reversed two piece mold out of alluminum to make wax plug to be ceramic dipped, fired then cast, with a ceramic casting shell you would just have to run the part through a fine tumbler and drill and tap the mounting holes.

Egggzactly

What he said