I made two receiver type brackets that are attached to the bumper. One is attached right where the bumper meets the frame and the other about 3+' away. They're clamped on with square U-bolts and also an 1-1/2" weld along the underside of the bracket.
I then fabed up a rack that slides into the two receiver brackets. It is held in place with two pins and also a top bolt to keep it from moving up and down.
I mounted two fork bike holders on opposite ends of the rack as well as an upright post each bike for that has the typical bike rubber strap holder.
I place the two bikes on facing opposite directions, forks clamped, frames rubber strapped and then set the two front dismounted tires in between the bikes. I then run a cable lock around all that and the ladder on the back of the 5er.
There's absolutely no way the bikes are coming off. They've been on the back of my 5er for going on 3 season and well over 11,000 miles. Plenty of rough roads as well.
They can't wiggle or bounce around. It's all in the way you have them secured.