Im going to steal a little of both your and Fenlander’s suggestions Jeff.
I cut two small PVC patch material circles. Drilled two tiny holes in one of the disk patches.. threaded fishing gut through the holes..then glued the other disk on the back so it seals the holes. Once Im ready ..warm this mushroom patch up..make the hole in the boat slightly larger... slobber the mushroom patch with glue..shove through the hole..then pull it up tight to the inside of the tube with the fishing gut.
Once it has set in position.. cut the gut flush..fill with sikaflex.. to level of black PVC then the external patch over the lot. That should sort it ..no problem..and jobs a done deal
Three minds working together .. are better than one gurnard fish head