What about drilling all the way through and using one piece of wood or stainless to connect the two holes. That would give you lots of support and if you were creative get the piece to blend in or contrast the new headliner so it looks like you did it on purpose...:)??? Use a thick enough piece so you can counter sink the bolts heads and cap them. I'd put the nut end in the cooler AND use some strapping to connect those two too..more support to the plastic in the cooler.