Hello Woody,
3 ideas:
Cobra seats are manufatured in the UK. I would contact the manufacturer if they still have original spare parts for the seats:
Cobra Seats - The World's Finest Automotive Seats
Ask a 3D printer shop if they can reproduce the parts. In general 3D printed parts are not that durable and have a rougher surface than injection molding plastic parts.
Use a hose that's profile is cutted exactly in the middle of the diameter and put it in the hole. There are some manufaturers that offer precut profile parts from rubber and other materials. This won't look like original part, but maybe will give a similar functionality.
Regards, Markus