Just to offer another alternative, here's what I use. Got em at Lowes.
I cut off the adhesive backing with a razor blade so it's just bare plastic. Then I two-part epoxied the mounts to the underside of my fenders. I use four zip ties per fender. I added a thin piece of foam tape to the stay to keep the zip ties from sliding along the stay.
Just did a 500 mile round trip, mostly highway at 70 mph and in the rain.. they didn't budge at all.