I've hauled a few yaks. Use a rail system to spread the surface area and give the system more contact. Use each available per drilled hole. Stainless hardware and lots of silicone sealant. On the inside, use the nut system that look like half golf balls. I drilled through my entire roof, there were not enough support ribs on the roof to secure the system. Mine has been up for 7 years, no issues. I have loaded 6 whitewater yaks up there at 50lbs each on a 7 western states tour.