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I also find that even just being used on metal trim pieces, nutserts/rivnuts tend to loosen over time. For example the lift up rear Aungar window louvers on my 1980 LC Lancer Hatchback have aluminium hinges fastened to the hatch with four nutserts (2 for each hinge) and I have to replace them every so often even though I probably lift the louvers up on the hinges less than once a year to thorough clean the rear window. They are also somewhat renowned for eventually spinning free if the bolt in them is frequently undone or removed; they are best for thing left permanently bolted together.

In any event, I hope despite my reservations, your innovative solution lasts.