tapping
Should you drill a pilot hole and no metal shavings come out, that means no metal is encapsulated in the deck. The next question is if the inner liner is solid against the deck of cabin top and if so, drill through for the bolt, caulk and use a larger washer when securing the nut. You may want to consider adding an acorn nut to so that if anyone should bump against the bolt, then they will not hit the sharp edge of the protrunign bolt. In the event that the space between the deck and cabin liner is not solid, then you will need to fill the area with for example liquid foam type or something. Another idea is to drill the hole inserting a stailess steel tube the length of the deck space so when the bolt comes through at the other end, you can use a space at the end of the bolt/tube and tighten with the nut. Again, use an acorn nut after installing the regular nut.Crazy Dave Condon