I must love doing these things.
Grommets that I ordered a week ago arrived today, in my mind I sort of figured a fix, hopefully it'll all be good. They were blank grommets so I had to cut a hole for the mount to fit through, of course drilling rubber is a proper pain so I was finally able to make use of the wad punches I bought a decade ago and made a lovely hole. You can see the old damaged fitting here, the heat from the old style incandescents would have weakened the plastic though to be fair in these times cubby is due to be melted down, but since it's working pretty well it'll have to wait a while.
The opening for the lamp is 15 mm and the grommets have a 13.9 mm fitting diameter, about as close to perfect as it needs to be. I put the white dot on to indicate the position of the LED, later I found I could see it fine through the glass so it isn't needed.
Now the sad part is I have to wait till nightfall so I can see the result before doing the work of wiring them properly. I removed the trim after having the misfortune of dropping a grommet when I was testing for fit, after much groping I removed the trim to get better access but after a long fruitless search including a hectare of ground surrounding the crime scene I happened to find it had dropped into my shirt pocket. The typical million to one shot that will happen whenever it isn't desired.