Best technique for the easy fix is remove the mounts from the car (6x17mm bolts) then use flanges or metal straps to secure the rubbers and mounts to the bumper with short sheet metal screws tapped into the bumper back.

Its an incredibly easy fix that just requires a little ingenuity. I used heavy cable hold down flanges like you see on telephone poles to fix mine.

And dont worry about 17mm bolts not lining up when you remount the bumper. The shock portion rotates and rubbers fit easily at only one location in the bumper back.