An energy absorber includes a base flange and elongated crush boxes that to form a substantially continuously face surface on the energy absorber. Back straps are attached across a back of the crush boxes to prevent unacceptable spreading of the side walls of the crush boxes during an impact. The back straps can be integrally formed when the energy absorber is molded, and can include a living hinge at one end and a free end that can be bent over and snap attached during post-molding assembly without additional secondary operations. Alternatively, a separate wall-stabilizing thermoformed component is sandwiched between the energy absorber and beam.