Dry fit the hub cap to the wheel, often it will locate precisely around the hubcap. Most 12mm screw caps have an internal diameter of 10mm on the back allowing them to sit neatly over existing hubcaps. 
 

If the central plastic cross (designed to locate into a cross head of a screw) prevents the cap from sitting flat on the wheel, remove it with the snips.
 

Carefully notch, chip and damage the screw cap with a sharp craft knife. Focus wear on the edges and consider applying more damage for wasteland/off-road vehicles. Little touches like repair patches and (pin vice drill) bullet holes can add visual interest and narrative. 
 

Apply super glue to the screw cap and carefully apply to the wheel ensuring it’s sitting centrally.