I added a set about a year ago. I tested the bumper before re-enforcing, by having my 230 lbs jump up and down on it, even at the ends.
The 4 squares attach about 1/4 of the bumper from each end. My bike carrier has two brackets spreading the weight/stress, out on the bumper.
Then I strap the bikes solid to the bike rack, and to the factory spare tire mounted to the back wall studs.
I couldn't get any shaking out of them if I wanted to.
I still check it at every stop, to make sure nothing is loosening up.
I believe the stress of them bouncing around would be the worst part.
Good luck and let us know how you make out.
2014 F 150 ecoboost
2008 Chrysler Aspen
09 Amerilite 21 (modified)
2013 Bendron 14' enclosed cargo
2011 4x8 open cargo