one more approach
I had to come out of my basement, so I wanted the airplane done and the steps removable. I put doublers inside, used flush rivets (4's if I remeber correctly). That puts all the rivets behind the step plate. I added nutplates to accept 4 AN 525 washer head screws, one in each corner. I think I used -10s. Removable and tidy. Regards, John.