I can think of 2 possibilities.
Put backing plates under headliner and through bolt. Will need to make as nice as possible. At least it's not in a spot where I'm likely to hit my head on fasteners.
Or embed into the deck some sort of stainless insert. If one removes as much core as possible with a allen wrench and fill with high strength filler thin enough to flow it can be very strong. Suspect if it failed it would pull big chunks of deck out. Done it and it's a bit tricky to do as alignment needs to be pretty much perfect. Would look better but is it strong enough?
