I beg to differ, my DNN fell off a few years ago and would not snap back on. I took it to my watchmaker and he glued it back on with epoxy. Told me he has seen plenty of instances such as this before. Perhaps when they left the factory they were interference fit, but mine was pretty loose.
Or if you don’t want to mess with it, buy a factory service bezel (which are cheap) and put that on and store the orginal away. The new bezel and fresh crystal on my “beater” made it look popping fresh and is completely reversible.
I exchanged the insert on a modern bezel fairly easy. Heated the whole bezel with a heat-gun for a short while. Did not take long before I could remove the insert with very little prying force using tweezers. The bezel was a bit warmer than was ok to touch without gloves but not near glowing or smoking hot.
Replaced insert and used a tiny amount of industrial glue.
Are you talking about the aluminium insert or the bezel itself?
