Here's pictures of how I did mine. Drill hole thru the "fins" on the back/bottom of the Pelican. I picked up a threaded metal rod at Home Depot. Just a touch longer than I really wanted, but pretty close. Thread the rod thru from side to side. If you get the hole size just right it will be nice and snug. You can cut off the excess on the ends and cap them off with acorn nuts, or Gorilla tape like I did because I was lazy. With this size box & rod, I can crank down plenty tight. I run one strap thru the rod side and one strap thru the handle, and the box doesn't budge on a side rail platform. I can easily pop the box open one handed to get the camera with no unstrapping. My camera box is a 1300 I believe, and I've modified a smaller 1120 the same way with a smaller diameter rod. I'm definitely more careful with strapping that one down. The big box has been in use for about 5-6 years now.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb