My rubber over plywood roof is heavy-duty enough to walk on, so I used short (2")piece of aluminum angle iron with a pair of stainless steel wood screws, sealed under and over with Dicor. A full description of my project and links to the photos can be found at VERY LONG - My 12V Electrical Modifications and Solar Power System Installation
Sorry, I don't have any fiberglass/luan experience, but maybe someone who does will pipe-in....
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter