-SG- The NOC has had a few rear carriers made for '24/25 Model 18s/16Hs, copying an original. The tubing is of the correct gauge and a carrier is shown fitted in one of the pictures attached. The carriers are available via The NOC shop on the Norton Owners Club website. There are just four left of the recent batch made and I am told the basic cost is £85 apiece. There are no plans to commission any more at present but if they sell as a consequence of going on the site, it may provide justification for the NOC to underwrite the production of some other rare/hard to find parts.
You could off course also make one yourself using these drawings.