I don't see it particularly as a Lucas, Prince of Darkness, issue. More that Morgan mount them in such a way that there is nothing to retain the brass threaded insert. That they fill up with water may be caused by the light unit (rather than the lens) being inverted when fitted. There is a slot (or there should be) in the base's edge that's there to allow the water to exit.
The same light units fitted in the late 1970s through to 1990s used unplated mild steel inserts that were fixed in position. These rusted thoroughly and meant that a bulb changing exercise was a real pain. I drilled these inserts out and replaced with brass.
Yes, by all means use a bit of tape to stop the insert dislocating. A better solution is to remove a small amount of the chromium plating around the edge of the insert hole with a Swiss file and solder the brass insert in situ.
I did this decades ago on my Morgans and have never had a problem since. I've even modified a few for friends.
