This old fellow can't find a pic ATM. Not difficult to imagine though. Cut hoop off & de-burr ends, find/turn on lathe two sections of thick walled pipe which need to be a virgin fit in the tubes. cut to length of over hang required to heavy 40 mm angle iron. A good tac weld required now to keep all in alignment , Take off car & complete welding, drilling for ball & electrics if needed. When the complete tow bar is re-fitted drill & bolt under boot.
I used a 50 mm/ M24 tail hitch so only needed a hole in the angle iron. If later used for a trailer I would suggest a 10 mm bracing bar (x2 ) from angle iron to top tube near roll-bar braces, threaded at base to impose a triangulation force on the hitch -- will prevent any bending force on the car chassis tubes where they terminate at cut off point.
Hope that helps.