Just the other week I witnessed a couple of "Jypsy-Bodymen" WELD a gaping bumper hole. They simply place the missing bumper part back into the hole (must have torn off in a parking lot incident). Holding the part so it would not move they ran a hot (wide-blade) soldering gun around the perimiter, fusing the parts together. This process was repeated from the underside. Some sanding, a little bondo, more sanding and you couldn't tell the difference. Nice little trick.