The frame for the two pieces was welded by a buddy, and he set up the bolts so they would match up to the structural ribs on top. They are thru bolted and the ends are covered inside by those nifty little plastic bolt covers you use on toilet tanks. No one has hit their head on them yet!
We've had 9 people up there for a photo op, and memorably, a friend rode up there with a GoPro all the way down the Transfaragasan in Romania. Probably not legal, but no harm, no foul.
The tire rack has lower sides, but have big bolts and bars that strap across the tires. These didn't budge, even in the crater-sized potholes we've encountered in our bus.