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I was able to thread M8x30 bolts by chamfering the tip so it will fit on the slots of the retainer.
Once you place the slotted tip of the screw, just start screwing and it will self thread. Even my daughters commented that the van felt more solid. Try to find some screws with a nice head and use a big washer.
Actually a very nice idea, but in my case the ones that span from one side to another in the middle of the headliner hold on tight. What I'm after are the ones on the outer edges front to back, they're bigger in diameter and have like 4 arms to fit in a bigger hole (~ 1").
If the middle ones ever fail on me, I set nutserts und use M8 bolts. I have about 50 or so nutsert in the van interior for whatever stuff needs to be mounted already.